Base JavaScript code position computation on JavaScript tracer.

pkg/compiler/lib/src/io/position_information.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// Source information system mapping that attempts a semantic mapping between
 /// offsets of JavaScript code points to offsets of Dart code points.
 
 library dart2js.source_information.position;
 
 import '../common.dart';
 import '../elements/elements.dart' show
     AstElement,
     LocalElement;
 import '../js/js.dart' as js;
 import '../js/js_source_mapping.dart';
 import '../js/js_debug.dart';
 import '../tree/tree.dart' show
     Node,
     Send;
 
+import 'code_output.dart' show
+    CodeBuffer;
 import 'source_file.dart';
 import 'source_information.dart';
 
 /// [SourceInformation] that consists of an offset position into the source
 /// code.
 class PositionSourceInformation extends SourceInformation {
   @override
   final SourceLocation startPosition;
 
   @override
@@ skipping 75 matching lines @@
 
   @override
   SourceInformationBuilder createBuilderForContext(AstElement element) {
     return new PositionSourceInformationBuilder(element);
   }
 
   @override
   SourceInformationProcessor createProcessor(SourceMapper mapper) {
     return new PositionSourceInformationProcessor(mapper);
   }
+
+  @override
+  void onComplete() {}
+
+  @override
+  SourceInformation buildSourceMappedMarker() {
+    return const SourceMappedMarker();
+  }
+}
+
+/// Marker used to tag the root nodes of source-mapped code.
+///
+/// This is needed to be able to distinguish JavaScript nodes that shouldn't
+/// have source locations (like the premable) from the nodes that should
+/// (like functions compiled from Dart code).
+class SourceMappedMarker extends SourceInformation {
+  const SourceMappedMarker();
+
+  @override
+  String get shortText => '';
+
+  @override
+  List<SourceLocation> get sourceLocations => const <SourceLocation>[];
+
+  @override
+  SourceSpan get sourceSpan => new SourceSpan(null, null, null);
 }
 
 /// [SourceInformationBuilder] that generates [PositionSourceInformation].
 class PositionSourceInformationBuilder implements SourceInformationBuilder {
   final SourceFile sourceFile;
   final String name;
 
   PositionSourceInformationBuilder(AstElement element)
       : sourceFile = element.implementation.compilationUnit.script.file,
         name = computeElementNameForSourceMaps(element);
@@ skipping 92 matching lines @@
   }
 }
 
 /// The start, end and closing offsets for a [js.Node].
 class CodePosition {
   final int startPosition;
   final int endPosition;
   final int closingPosition;
 
   CodePosition(this.startPosition, this.endPosition, this.closingPosition);
+
+  int getPosition(CodePositionKind kind) {
+    switch (kind) {
+      case CodePositionKind.START:
+        return startPosition;
+      case CodePositionKind.END:
+        return endPosition;
+      case CodePositionKind.CLOSING:
+        return closingPosition;
+    }
+  }
+
+  String toString() {
+    return 'CodePosition(start=$startPosition,'
+           'end=$endPosition,closing=$closingPosition)';
+  }
+}
+
+/// A map from a [js.Node] to its [CodePosition].
+abstract class CodePositionMap {
+  CodePosition operator [](js.Node node);
 }
 
 /// Registry for mapping [js.Node]s to their [CodePosition].
-class CodePositionRecorder {
+class CodePositionRecorder implements CodePositionMap {
   Map<js.Node, CodePosition> _codePositionMap =
       new Map<js.Node, CodePosition>.identity();
 
   void registerPositions(js.Node node,
                          int startPosition,
                          int endPosition,
                          int closingPosition) {
     registerCodePosition(node,
         new CodePosition(startPosition, endPosition, closingPosition));
   }
 
   void registerCodePosition(js.Node node, CodePosition codePosition) {
     _codePositionMap[node] = codePosition;
   }
 
   CodePosition operator [](js.Node node) => _codePositionMap[node];
 }
 
+/// Enum values for the part of a Dart node used for the source location offset.
 enum SourcePositionKind {
+  /// The source mapping should point to the start of the Dart node.
+  ///
+  /// For instance the first '(' for the `(*)()` call and 'f' of both the
+  /// `foo()` and the `*.bar()` call:
+  ///
+  ///     (foo().bar())()
+  ///     ^                       // the start of the `(*)()` node
+  ///      ^                      // the start of the `foo()` node
+  ///      ^                      // the start of the `*.bar()` node
+  ///
   START,
+
+  /// The source mapping should point an inner position of the Dart node.
+  ///
+  /// For instance the second '(' of the `(*)()` call, the 'f' of the `foo()`
+  /// call and the 'b' of the `*.bar()` call:
+  ///
+  ///     (foo().bar())()
+  ///                  ^          // the inner position of the `(*)()` node
+  ///      ^                      // the inner position of the `foo()` node
+  ///            ^                // the inner position of the `*.bar()` node
+  ///
+  /// For function expressions the inner position is the closing brace or the
+  /// arrow:
+  ///
+  ///     foo() => () {}
+  ///           ^                 // the inner position of the 'foo' function
+  ///                  ^          // the inner position of the closure
+  ///
+  INNER,
+}
+
+SourceLocation getSourceLocation(
+    SourceInformation sourceInformation,
+    [SourcePositionKind sourcePositionKind = SourcePositionKind.START]) {
+  if (sourceInformation == null) return null;
+  switch (sourcePositionKind) {
+    case SourcePositionKind.START:
+      return sourceInformation.startPosition;
+    case SourcePositionKind.INNER:
+      return sourceInformation.closingPosition;
+  }
+}
+
+/// Enum values for the part of the JavaScript node used for the JavaScript
+/// code offset of a source mapping.
+enum CodePositionKind {
+  /// The source mapping is put on left-most offset of the node.
+  ///
+  /// For instance on the 'f' of a function or 'r' of a return statement:
+  ///
+  ///     foo: function() { return 0; }
+  ///          ^                              // the function start position
+  ///                       ^                 // the return start position
+  START,
+
+  /// The source mapping is put on the closing token.
+  ///
+  /// For instance on the '}' of a function or the ';' of a return statement:
+  ///
+  ///     foo: function() { return 0; }
+  ///                                 ^       // the function closing position
+  ///                               ^         // the return closing position
+  ///
   CLOSING,
-  END,
-}
-
-enum CodePositionKind {
-  START,
-  CLOSING,
+
+  /// The source mapping is put at the end of the code for the node.
+  ///
+  /// For instance after '}' of a function or after the ';' of a return
+  /// statement:
+  ///
+  ///     foo: function() { return 0; }
+  ///                                  ^       // the function end position
+  ///                                ^         // the return end position
+  ///
   END,
 }
 
 /// Processor that associates [SourceLocation]s from [SourceInformation] on
 /// [js.Node]s with the target offsets in a [SourceMapper].
-class PositionSourceInformationProcessor
-    extends js.BaseVisitor implements SourceInformationProcessor {
-  final CodePositionRecorder codePositions = new CodePositionRecorder();
-  final SourceMapper sourceMapper;
-
-  PositionSourceInformationProcessor(this.sourceMapper);
-
-  void process(js.Node node) {
-    node.accept(this);
-  }
-
-  void visitChildren(js.Node node) {
-    node.visitChildren(this);
-  }
-
-  CodePosition getCodePosition(js.Node node) {
-    return codePositions[node];
-  }
-
-  /// Associates [sourceInformation] with the JavaScript [node].
-  ///
-  /// The offset into the JavaScript code is computed by pulling the
-  /// [codePositionKind] from the code positions associated with
-  /// [codePositionNode].
-  ///
-  /// The mapped Dart source location is computed by pulling the
-  /// [sourcePositionKind] source location from [sourceInformation].
-  void apply(js.Node node,
-             js.Node codePositionNode,
-             CodePositionKind codePositionKind,
-             SourceInformation sourceInformation,
-             SourcePositionKind sourcePositionKind) {
-    if (sourceInformation != null) {
-      CodePosition codePosition = getCodePosition(codePositionNode);
-      // We should always have recorded the needed code positions.
-      assert(invariant(
-          NO_LOCATION_SPANNABLE,
-          codePosition != null,
-          message:
-            "Code position missing for "
-            "${nodeToString(codePositionNode)}:\n"
-            "${DebugPrinter.prettyPrint(node)}"));
-      if (codePosition == null) return;
-      int codeLocation;
-      SourceLocation sourceLocation;
-      switch (codePositionKind) {
-        case CodePositionKind.START:
-          codeLocation = codePosition.startPosition;
-          break;
-        case CodePositionKind.CLOSING:
-          codeLocation = codePosition.closingPosition;
-          break;
-        case CodePositionKind.END:
-          codeLocation = codePosition.endPosition;
-          break;
-      }
-      switch (sourcePositionKind) {
-        case SourcePositionKind.START:
-          sourceLocation = sourceInformation.startPosition;
-          break;
-        case SourcePositionKind.CLOSING:
-          sourceLocation = sourceInformation.closingPosition;
-          break;
-        case SourcePositionKind.END:
-          sourceLocation = sourceInformation.endPosition;
-          break;
-      }
-      if (codeLocation != null && sourceLocation != null) {
-        sourceMapper.register(node, codeLocation, sourceLocation);
-      }
-    }
-  }
-
-  @override
-  visitNode(js.Node node) {
-    SourceInformation sourceInformation = node.sourceInformation;
-    if (sourceInformation != null) {
-      /// Associates the left-most position of the JS code with the left-most
-      /// position of the Dart code.
-      apply(node,
-          node, CodePositionKind.START,
-          sourceInformation, SourcePositionKind.START);
-    }
-    visitChildren(node);
-  }
-
-  @override
-  visitFun(js.Fun node) {
-    SourceInformation sourceInformation = node.sourceInformation;
-    if (sourceInformation != null) {
-      /// Associates the end brace of the JavaScript function with the end brace
-      /// of the Dart function (or the `;` in case of arrow notation).
-      apply(node,
-          node, CodePositionKind.CLOSING,
-          sourceInformation, SourcePositionKind.CLOSING);
-    }
-
-    visitChildren(node);
-  }
-
-  @override
-  visitExpressionStatement(js.ExpressionStatement node) {
-    visitChildren(node);
-  }
-
-  @override
-  visitBinary(js.Binary node) {
-    visitChildren(node);
-  }
-
-  @override
-  visitAccess(js.PropertyAccess node) {
-    visitChildren(node);
-  }
-
-  @override
-  visitCall(js.Call node) {
-    SourceInformation sourceInformation = node.sourceInformation;
-    if (sourceInformation != null) {
-      if (node.target is js.PropertyAccess) {
-        js.PropertyAccess access = node.target;
-        js.Node target = access;
-        bool pureAccess = false;
-        while (target is js.PropertyAccess) {
-          js.PropertyAccess targetAccess = target;
-          if (targetAccess.receiver is js.VariableUse ||
-              targetAccess.receiver is js.This) {
-            pureAccess = true;
-            break;
-          } else {
-            target = targetAccess.receiver;
-          }
-        }
-        if (pureAccess) {
-          // a.m()   this.m()  a.b.c.d.m()
-          // ^       ^         ^
-          apply(
-              node,
-              node,
-              CodePositionKind.START,
-              sourceInformation,
-              SourcePositionKind.START);
-        } else {
-          // *.m()  *.a.b.c.d.m()
-          //   ^              ^
-          apply(
-              node,
-              access.selector,
-              CodePositionKind.START,
-              sourceInformation,
-              SourcePositionKind.CLOSING);
-        }
-      } else if (node.target is js.VariableUse) {
-        // m()
-        // ^
-        apply(
-            node,
-            node,
-            CodePositionKind.START,
-            sourceInformation,
-            SourcePositionKind.START);
-      } else if (node.target is js.Fun || node.target is js.New) {
-        // function(){}()  new Function("...")()
-        //             ^                      ^
-        apply(
-            node,
-            node.target,
-            CodePositionKind.END,
-            sourceInformation,
-            SourcePositionKind.CLOSING);
-      } else {
-        assert(invariant(NO_LOCATION_SPANNABLE, false,
-            message: "Unexpected property access ${nodeToString(node)}:\n"
-                     "${DebugPrinter.prettyPrint(node)}"));
-        // Don't know....
-        apply(
-            node,
-            node,
-            CodePositionKind.START,
-            sourceInformation,
-            SourcePositionKind.START);
-      }
-    }
-    visitChildren(node);
+class PositionSourceInformationProcessor implements SourceInformationProcessor {
+  final CodePositionRecorder codePositionRecorder = new CodePositionRecorder();
+  CodePositionMap codePositionMap;
+  List<TraceListener> traceListeners;
+
+  PositionSourceInformationProcessor(
+      SourceMapper sourceMapper,
+      [Coverage coverage]) {
+    codePositionMap = coverage != null
+              ? new CodePositionCoverage(codePositionRecorder, coverage)
+              : codePositionRecorder;
+    traceListeners = [new PositionTraceListener(sourceMapper)];
+    if (coverage != null) {
+      traceListeners.add(new CoverageListener(coverage));
+    }
+  }
+
+  void process(js.Node node, CodeBuffer codeBuffer) {
+    new JavaScriptTracer(codePositionMap, traceListeners).apply(node);
   }
 
   @override
   void onPositions(js.Node node,
                    int startPosition,
                    int endPosition,
                    int closingPosition) {
-    codePositions.registerPositions(
+    codePositionRecorder.registerPositions(
         node, startPosition, endPosition, closingPosition);
   }
 }
+
+/// [TraceListener] that register [SourceLocation]s with a [SourceMapper].
+class PositionTraceListener extends TraceListener {
+  final SourceMapper sourceMapper;
+
+  PositionTraceListener(this.sourceMapper);
+
+  @override
+  void onStep(js.Node node, Offset offset, StepKind kind) {
+    SourceInformation sourceInformation = node.sourceInformation;
+    if (sourceInformation == null) return;
+
+    SourcePositionKind sourcePositionKind = SourcePositionKind.START;
+    switch (kind) {
+      case StepKind.FUN:
+        sourcePositionKind = SourcePositionKind.INNER;
+        break;
+      case StepKind.CALL:
+        CallPosition callPosition =
+            CallPosition.getSemanticPositionForCall(node);
+        sourcePositionKind = callPosition.sourcePositionKind;
+        break;
+      case StepKind.NEW:
+      case StepKind.RETURN:
+      case StepKind.BREAK:
+      case StepKind.CONTINUE:
+      case StepKind.THROW:
+      case StepKind.EXPRESSION_STATEMENT:
+      case StepKind.IF_CONDITION:
+      case StepKind.FOR_INITIALIZER:
+      case StepKind.FOR_CONDITION:
+      case StepKind.FOR_UPDATE:
+      case StepKind.WHILE_CONDITION:
+      case StepKind.DO_CONDITION:
+      case StepKind.SWITCH_EXPRESSION:
+        break;
+    }
+    int codeLocation = offset.codeOffset;
+    SourceLocation sourceLocation =
+        getSourceLocation(sourceInformation, sourcePositionKind);
+    if (codeLocation != null && sourceLocation != null) {
+      sourceMapper.register(node, codeLocation, sourceLocation);
+    }
+  }
+}
+
+/// The position of a [js.Call] node.
+class CallPosition {
+  final js.Node node;
+  final CodePositionKind codePositionKind;
+  final SourcePositionKind sourcePositionKind;
+
+  CallPosition(this.node, this.codePositionKind, this.sourcePositionKind);
+
+  /// Computes the [CallPosition] for [node].
+  static CallPosition getSemanticPositionForCall(js.Call node) {
+    if (node.target is js.PropertyAccess) {
+      js.PropertyAccess access = node.target;
+      js.Node target = access;
+      bool pureAccess = false;
+      while (target is js.PropertyAccess) {
+        js.PropertyAccess targetAccess = target;
+        if (targetAccess.receiver is js.VariableUse ||
+            targetAccess.receiver is js.This) {
+          pureAccess = true;
+          break;
+        } else {
+          target = targetAccess.receiver;
+        }
+      }
+      if (pureAccess) {
+        // a.m()   this.m()  a.b.c.d.m()
+        // ^       ^         ^
+        return new CallPosition(
+            node,
+            CodePositionKind.START,
+            SourcePositionKind.START);
+      } else {
+        // *.m()  *.a.b.c.d.m()
+        //   ^              ^
+        return new CallPosition(
+            access.selector,
+            CodePositionKind.START,
+            SourcePositionKind.INNER);
+      }
+    } else if (node.target is js.VariableUse) {
+      // m()
+      // ^
+      return new CallPosition(
+          node,
+          CodePositionKind.START,
+          SourcePositionKind.START);
+    } else if (node.target is js.Fun || node.target is js.New) {
+      // function(){}()  new Function("...")()
+      //             ^                      ^
+      return new CallPosition(
+          node.target,
+          CodePositionKind.END,
+          SourcePositionKind.INNER);
+    } else if (node.target is js.Binary || node.target is js.Call) {
+      // (0,a)()   m()()
+      //      ^       ^
+      return new CallPosition(
+          node.target,
+          CodePositionKind.END,
+          SourcePositionKind.INNER);
+    } else {
+      assert(invariant(NO_LOCATION_SPANNABLE, false,
+          message: "Unexpected property access ${nodeToString(node)}:\n"
+                   "${DebugPrinter.prettyPrint(node)}"));
+      // Don't know....
+      return new CallPosition(
+          node,
+          CodePositionKind.START,
+          SourcePositionKind.START);
+    }
+  }
+}
+
+class Offset {
+  /// The offset of the enclosing statement relative to the beginning of the
+  /// file.
+  ///
+  /// For instance:
+  ///
+  ///     foo().bar(baz());
+  ///     ^                  // the statement offset of the `foo()` call
+  ///     ^                  // the statement offset of the `*.bar()` call
+  ///     ^                  // the statement offset of the `baz()` call
+  ///
+  final int statementOffset;
+
+  /// The `subexpression` offset of the step. This is the (mostly) unique
+  /// offset relative to the beginning of the file, that identifies the
+  /// current of execution.
+  ///
+  /// For instance:
+  ///
+  ///     foo().bar(baz());
+  ///     ^                   // the subexpression offset of the `foo()` call
+  ///           ^             // the subexpression offset of the `*.bar()` call
+  ///               ^         // the subexpression offset of the `baz()` call
+  ///
+  /// Here, even though the JavaScript node for the `*.bar()` call contains
+  /// the `foo()` its execution is identified by the `bar` identifier more than
+  /// the foo identifier.
+  ///
+  final int codeOffset;
+
+  /// The `left-to-right` offset of the step. This is a restricted version of
+  /// [subexpressionOffset] where offset by the execution order are always

[Siggi Cherem (dart-lang), 2016/01/22 19:32:58]

A few notes:
 - [subexpressionOffset] => [codeOffset]?
   Or rename codeOffset above to subexpressionOffset? (I slightly prefer
renaming)

 - grammar (not sure I follow "where offset by the execution...") Suggestion:

  "This is like [subexpressionOffset] but restricted so that the offset of each
subexpression in execution order is monotonically increasing."

[Johnni Winther, 2016/01/25 09:58:55]

Done.

+  /// increasing within the same statement.
+  ///
+  /// For instance:
+  ///
+  ///     foo().bar(baz());
+  ///     ^                   // the left-to-right offset of the `foo()` call
+  ///           ^             // the left-to-right offset of the `*.bar()` call
+  ///     ^                   // the left-to-right offset of the `baz()` call
+  ///
+  /// Here, `baz()` is executed before `foo()` be needs to use 'f' as its best

[Siggi Cherem (dart-lang), 2016/01/22 19:32:58]

nit: "be needs" => "so we need to"?

[Johnni Winther, 2016/01/25 09:58:55]

Done.

+  /// position under the restriction.
+  ///
+  final int leftToRightOffset;

[Siggi Cherem (dart-lang), 2016/01/22 19:32:58]

another idea for the name here: executionOrderOffset

[Johnni Winther, 2016/01/25 09:58:55]

I find that equally confusing :(

+
+  Offset(this.statementOffset, this.leftToRightOffset, this.codeOffset);
+
+  String toString() {
+    return 'Offset[statementOffset=$statementOffset,'
+        'leftToRightOffset=$leftToRightOffset,'
+        'codeOffset=$codeOffset]';
+  }
+}
+
+enum BranchKind {
+  CONDITION,
+  LOOP,
+  CATCH,
+  FINALLY,
+  CASE,
+}
+
+enum StepKind {
+  FUN,
+  CALL,
+  NEW,
+  RETURN,
+  BREAK,
+  CONTINUE,
+  THROW,
+  EXPRESSION_STATEMENT,
+  IF_CONDITION,
+  FOR_INITIALIZER,
+  FOR_CONDITION,
+  FOR_UPDATE,
+  WHILE_CONDITION,
+  DO_CONDITION,
+  SWITCH_EXPRESSION,
+}
+
+/// Listener for the [JavaScriptTracer].
+abstract class TraceListener {
+  /// Called before [root] node is procesed by the [JavaScriptTracer].
+  void onStart(js.Node root) {}
+
+  /// Called after [root] node has been procesed by the [JavaScriptTracer].
+  void onEnd(js.Node root) {}
+
+  /// Called when a branch of the given [kind] is started. [value] is provided
+  /// to distinguish true/false branches of [BranchKind.CONDITION] and cases of
+  /// [Branch.CASE].
+  void pushBranch(BranchKind kind, [value]) {}
+
+  /// Called when the current branch ends.
+  void popBranch() {}
+
+  /// Called when [node] defines a step of the given [kind] at the given
+  /// [offset] when the generated JavaScript code.
+  void onStep(js.Node node, Offset offset, StepKind kind) {}
+}
+
+/// Visitor that computes the [js.Node]s the are part of the JavaScript
+/// steppable execution and thus needs source mapping locations.
+class JavaScriptTracer extends js.BaseVisitor  {
+  final CodePositionMap codePositions;
+  final List<TraceListener> listeners;
+
+  /// The steps added by subexpressions.
+  List steps = [];
+
+  /// The offset of the current statement.
+  int statementOffset;
+
+  /// The current offset in left-to-right progression.
+  int leftToRightOffset;
+
+  /// The offset of the surrounding statement, used for the first subexpression.
+  int offsetPosition;
+
+  bool active;
+
+  JavaScriptTracer(this.codePositions,
+           this.listeners,
+           {this.active: false});
+
+  void notifyStart(js.Node node) {
+    listeners.forEach((listener) => listener.onStart(node));
+  }
+
+  void notifyEnd(js.Node node) {
+    listeners.forEach((listener) => listener.onEnd(node));
+  }
+
+  void notifyPushBranch(BranchKind kind, [value]) {
+    if (active) {
+      listeners.forEach((listener) => listener.pushBranch(kind, value));
+    }
+  }
+
+  void notifyPopBranch() {
+    if (active) {
+      listeners.forEach((listener) => listener.popBranch());
+    }
+  }
+
+  void notifyStep(js.Node node, Offset offset, StepKind kind) {
+    if (active) {
+      listeners.forEach((listener) => listener.onStep(node, offset, kind));
+    }
+  }
+
+  void apply(js.Node node) {
+    notifyStart(node);
+    node.accept(this);
+    notifyEnd(node);
+  }
+
+  @override
+  visitNode(js.Node node) {
+    node.visitChildren(this);
+  }
+
+  visit(js.Node node, [BranchKind branch, value]) {
+    if (node != null) {
+      if (branch != null) {
+        notifyPushBranch(branch, value);
+        node.accept(this);
+        notifyPopBranch();
+      } else {
+        node.accept(this);
+      }
+    }
+  }
+
+  visitList(List<js.Node> nodeList) {
+    if (nodeList != null) {
+      for (js.Node node in nodeList) {
+        visit(node);
+      }
+    }
+  }
+
+  @override
+  visitFun(js.Fun node) {
+    bool activeBefore = active;
+    if (!active) {
+      active = node.sourceInformation != null;
+    }
+    visit(node.body);
+    leftToRightOffset = statementOffset =
+        getSyntaxOffset(node, kind: CodePositionKind.CLOSING);
+    Offset offset = getOffsetForNode(node, statementOffset);
+    notifyStep(node, offset, StepKind.FUN);
+    active = activeBefore;
+  }
+
+  @override
+  visitBlock(js.Block node) {
+    for (js.Statement statement in node.statements) {
+      visit(statement);
+    }
+  }
+
+  int getSyntaxOffset(js.Node node,
+                      {CodePositionKind kind: CodePositionKind.START}) {
+    CodePosition codePosition = codePositions[node];
+    if (codePosition != null) {
+      return codePosition.getPosition(kind);
+    }
+    return null;
+  }
+
+  visitSubexpression(js.Node parent,
+                     js.Expression child,
+                     int codeOffset,
+                     StepKind kind) {
+    var oldSteps = steps;
+    steps = [];
+    offsetPosition = codeOffset;
+    visit(child);
+    if (steps.isEmpty) {
+      notifyStep(parent,
+          getOffsetForNode(parent, offsetPosition),
+          kind);
+    }
+    steps = oldSteps;
+  }
+
+  @override
+  visitExpressionStatement(js.ExpressionStatement node) {
+    statementOffset = getSyntaxOffset(node);
+    visitSubexpression(
+        node, node.expression, statementOffset,
+        StepKind.EXPRESSION_STATEMENT);
+    statementOffset = null;
+    leftToRightOffset = null;
+  }
+
+  @override
+  visitEmptyStatement(js.EmptyStatement node) {}
+
+  @override
+  visitCall(js.Call node) {
+    visit(node.target);
+    int oldPosition = offsetPosition;
+    offsetPosition = null;
+    visitList(node.arguments);
+    offsetPosition = oldPosition;
+    CallPosition callPosition =
+        CallPosition.getSemanticPositionForCall(node);
+    js.Node positionNode = callPosition.node;
+    int callOffset = getSyntaxOffset(
+        positionNode, kind: callPosition.codePositionKind);
+    if (offsetPosition == null) {
+      offsetPosition = callOffset;
+    }
+    Offset offset = getOffsetForNode(positionNode, offsetPosition);
+    notifyStep(node, offset, StepKind.CALL);
+    steps.add(node);
+    offsetPosition = null;
+  }
+
+  @override
+  visitNew(js.New node) {
+    visit(node.target);
+    visitList(node.arguments);
+    notifyStep(
+        node, getOffsetForNode(node, getSyntaxOffset(node)), StepKind.NEW);
+    steps.add(node);
+    offsetPosition = null;
+  }
+
+  @override
+  visitAccess(js.PropertyAccess node) {
+    visit(node.receiver);
+    visit(node.selector);
+  }
+
+  @override
+  visitVariableUse(js.VariableUse node) {}
+
+  @override
+  visitLiteralBool(js.LiteralBool node) {}
+
+  @override
+  visitLiteralString(js.LiteralString node) {}
+
+  @override
+  visitLiteralNumber(js.LiteralNumber node) {}
+
+  @override
+  visitLiteralNull(js.LiteralNull node) {}
+
+  @override
+  visitName(js.Name node) {}
+
+  @override
+  visitVariableDeclarationList(js.VariableDeclarationList node) {
+    visitList(node.declarations);
+  }
+
+  @override
+  visitVariableDeclaration(js.VariableDeclaration node) {}
+
+  @override
+  visitVariableInitialization(js.VariableInitialization node) {
+    visit(node.leftHandSide);
+    visit(node.value);
+  }
+
+  @override
+  visitAssignment(js.Assignment node) {
+    visit(node.leftHandSide);
+    visit(node.value);
+  }
+
+  @override
+  visitIf(js.If node) {
+    statementOffset = getSyntaxOffset(node);
+    visitSubexpression(node, node.condition, statementOffset,
+        StepKind.IF_CONDITION);
+    statementOffset = null;
+    visit(node.then, BranchKind.CONDITION, true);
+    visit(node.otherwise, BranchKind.CONDITION, false);
+  }
+
+  @override
+  visitFor(js.For node) {
+    int offset = statementOffset = getSyntaxOffset(node);
+    statementOffset = offset;
+    leftToRightOffset = null;
+    if (node.init != null) {
+      visitSubexpression(node, node.init, getSyntaxOffset(node),
+          StepKind.FOR_INITIALIZER);
+    }
+
+    if (node.condition != null) {
+      visitSubexpression(node, node.condition, getSyntaxOffset(node.condition),
+          StepKind.FOR_CONDITION);
+    }
+
+    notifyPushBranch(BranchKind.LOOP);
+    visit(node.body);
+
+    statementOffset = offset;
+    if (node.update != null) {
+      visitSubexpression(node, node.update, getSyntaxOffset(node.update),
+          StepKind.FOR_UPDATE);
+    }
+
+    notifyPopBranch();
+  }
+
+  @override
+  visitWhile(js.While node) {
+    statementOffset = getSyntaxOffset(node);
+    if (node.condition != null) {
+      visitSubexpression(node, node.condition, getSyntaxOffset(node.condition),
+          StepKind.WHILE_CONDITION);
+    }
+    statementOffset = null;
+    leftToRightOffset = null;
+
+    visit(node.body, BranchKind.LOOP);
+  }
+
+  @override
+  visitDo(js.Do node) {
+    statementOffset = getSyntaxOffset(node);
+    visit(node.body);
+    if (node.condition != null) {
+      visitSubexpression(node, node.condition, getSyntaxOffset(node.condition),
+          StepKind.DO_CONDITION);
+    }
+    statementOffset = null;
+    leftToRightOffset = null;
+  }
+
+  @override
+  visitBinary(js.Binary node) {
+    visit(node.left);
+    visit(node.right);
+  }
+
+  @override
+  visitThis(js.This node) {}
+
+  @override
+  visitReturn(js.Return node) {
+    statementOffset = getSyntaxOffset(node);
+    visit(node.value);
+    notifyStep(
+        node, getOffsetForNode(node, getSyntaxOffset(node)), StepKind.RETURN);
+    statementOffset = null;
+    leftToRightOffset = null;
+  }
+
+  @override
+  visitThrow(js.Throw node) {
+    statementOffset = getSyntaxOffset(node);
+    visit(node.expression);
+    notifyStep(
+        node, getOffsetForNode(node, getSyntaxOffset(node)), StepKind.THROW);
+    statementOffset = null;
+    leftToRightOffset = null;
+  }
+
+  @override
+  visitContinue(js.Continue node) {
+    statementOffset = getSyntaxOffset(node);
+    notifyStep(
+        node, getOffsetForNode(node, getSyntaxOffset(node)), StepKind.CONTINUE);
+    statementOffset = null;
+    leftToRightOffset = null;
+  }
+
+  @override
+  visitBreak(js.Break node) {
+    statementOffset = getSyntaxOffset(node);
+    notifyStep(
+        node, getOffsetForNode(node, getSyntaxOffset(node)), StepKind.BREAK);
+    statementOffset = null;
+    leftToRightOffset = null;
+  }
+
+  @override
+  visitTry(js.Try node) {
+    visit(node.body);
+    visit(node.catchPart, BranchKind.CATCH);
+    visit(node.finallyPart, BranchKind.FINALLY);
+  }
+
+  @override
+  visitCatch(js.Catch node) {
+    visit(node.body);
+  }
+
+  @override
+  visitConditional(js.Conditional node) {
+    visit(node.condition);
+    visit(node.then, BranchKind.CONDITION, true);
+    visit(node.otherwise, BranchKind.CONDITION, false);
+  }
+
+  @override
+  visitPrefix(js.Prefix node) {
+    visit(node.argument);
+  }
+
+  @override
+  visitPostfix(js.Postfix node) {
+    visit(node.argument);
+  }
+
+  @override
+  visitObjectInitializer(js.ObjectInitializer node) {
+    visitList(node.properties);
+  }
+
+  @override
+  visitProperty(js.Property node) {
+    visit(node.name);
+    visit(node.value);
+  }
+
+  @override
+  visitRegExpLiteral(js.RegExpLiteral node) {}
+
+  @override
+  visitSwitch(js.Switch node) {
+    statementOffset = getSyntaxOffset(node);
+    visitSubexpression(node, node.key, getSyntaxOffset(node),
+        StepKind.SWITCH_EXPRESSION);
+    statementOffset = null;
+    leftToRightOffset = null;
+    for (int i = 0; i < node.cases.length; i++) {
+      visit(node.cases[i], BranchKind.CASE, i);
+    }
+  }
+
+  @override
+  visitCase(js.Case node) {
+    visit(node.expression);
+    visit(node.body);
+  }
+
+  @override
+  visitDefault(js.Default node) {
+    visit(node.body);
+  }
+
+  @override
+  visitArrayInitializer(js.ArrayInitializer node) {
+    visitList(node.elements);
+  }
+
+  @override
+  visitArrayHole(js.ArrayHole node) {}
+
+  @override
+  visitLabeledStatement(js.LabeledStatement node) {
+    statementOffset = getSyntaxOffset(node);
+    visit(node.body);
+    statementOffset = null;
+  }
+
+  Offset getOffsetForNode(js.Node node, int codeOffset) {
+    if (codeOffset == null) {
+      CodePosition codePosition = codePositions[node];
+      if (codePosition != null) {
+        codeOffset = codePosition.startPosition;
+      }
+    }
+    if (leftToRightOffset != null && leftToRightOffset < codeOffset) {
+      leftToRightOffset = codeOffset;
+    }
+    if (leftToRightOffset == null) {
+      leftToRightOffset = statementOffset;
+    }
+    return new Offset(statementOffset, leftToRightOffset, codeOffset);
+  }
+}
+
+
+class Coverage {
+  Set<js.Node> _nodesWithInfo = new Set<js.Node>();
+  int _nodesWithInfoCount = 0;
+  Set<js.Node> _nodesWithoutInfo = new Set<js.Node>();
+  int _nodesWithoutInfoCount = 0;
+  Map<Type, int> _nodesWithoutInfoCountByType = <Type, int>{};
+  Set<js.Node> _nodesWithoutOffset = new Set<js.Node>();
+  int _nodesWithoutOffsetCount = 0;
+
+  void registerNodeWithInfo(js.Node node) {
+    _nodesWithInfo.add(node);
+  }
+
+  void registerNodeWithoutInfo(js.Node node) {
+    _nodesWithoutInfo.add(node);
+  }
+
+  void registerNodesWithoutOffset(js.Node node) {
+    _nodesWithoutOffset.add(node);
+  }
+
+  void collapse() {
+    _nodesWithInfoCount += _nodesWithInfo.length;
+    _nodesWithInfo.clear();
+    _nodesWithoutOffsetCount += _nodesWithoutOffset.length;
+    _nodesWithoutOffset.clear();
+
+    _nodesWithoutInfoCount += _nodesWithoutInfo.length;
+    for (js.Node node in _nodesWithoutInfo) {
+      if (node is js.ExpressionStatement) {
+        _nodesWithoutInfoCountByType.putIfAbsent(
+            node.expression.runtimeType, () => 0);
+        _nodesWithoutInfoCountByType[node.expression.runtimeType]++;
+      } else {
+        _nodesWithoutInfoCountByType.putIfAbsent(
+            node.runtimeType, () => 0);
+        _nodesWithoutInfoCountByType[node.runtimeType]++;
+      }
+    }
+    _nodesWithoutInfo.clear();
+  }
+
+  String getCoverageReport() {
+    collapse();
+    StringBuffer sb = new StringBuffer();
+    int total = _nodesWithInfoCount + _nodesWithoutInfoCount;
+    if (total > 0) {
+      sb.write(_nodesWithoutInfoCount);
+      sb.write('/');
+      sb.write(total);
+      sb.write(' (');
+      sb.write((100.0 * _nodesWithInfoCount / total).toStringAsFixed(2));
+      sb.write('%) nodes with info.');
+    } else {
+      sb.write('No nodes.');
+    }
+    if (_nodesWithoutOffsetCount > 0) {
+      sb.write(' ');
+      sb.write(_nodesWithoutOffsetCount);
+      sb.write(' node');
+      if (_nodesWithoutOffsetCount > 1) {
+        sb.write('s');
+      }
+      sb.write(' without offset.');
+    }
+    if (_nodesWithoutInfoCount > 0) {
+      sb.write('\nNodes without info (');
+      sb.write(_nodesWithoutInfoCount);
+      sb.write(') by runtime type:');
+      _nodesWithoutInfoCountByType.forEach((Type type, int count) {
+        sb.write('\n ');
+        sb.write(count);
+        sb.write(' ');
+        sb.write(type);
+        sb.write(' node');
+        if (count > 1) {
+          sb.write('s');
+        }
+      });
+      sb.write('\n');
+    }
+    return sb.toString();
+  }
+
+  String toString() => getCoverageReport();
+}
+
+/// [TraceListener] that registers [onStep] callbacks with [coverage].
+class CoverageListener extends TraceListener {
+  final Coverage coverage;
+
+  CoverageListener(this.coverage);
+
+  @override
+  void onStep(js.Node node, Offset offset, StepKind kind) {
+    SourceInformation sourceInformation = node.sourceInformation;
+    if (sourceInformation != null) {
+      coverage.registerNodeWithInfo(node);
+    } else {
+      coverage.registerNodeWithoutInfo(node);
+    }
+  }
+
+  @override
+  void onEnd(js.Node node) {
+    coverage.collapse();
+  }
+}
+
+/// [CodePositionMap] that registers calls with [Coverage].
+class CodePositionCoverage implements CodePositionMap {
+  final CodePositionMap codePositions;
+  final Coverage coverage;
+
+  CodePositionCoverage(this.codePositions, this.coverage);
+
+  @override
+  CodePosition operator [](js.Node node) {
+    CodePosition codePosition = codePositions[node];
+    if (codePosition == null) {
+      coverage.registerNodesWithoutOffset(node);
+   }
+    return codePosition;
+  }
+}