Base JavaScript code position computation on JavaScript tracer.

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

Index: pkg/compiler/lib/src/io/position_information.dart
diff --git a/pkg/compiler/lib/src/io/position_information.dart b/pkg/compiler/lib/src/io/position_information.dart
index 0e384c32bc3fd51ef76fc6950110ef51381e4dff..14c8163c08b6af887c59940ca01aa1a33fb2c128 100644
--- a/pkg/compiler/lib/src/io/position_information.dart
+++ b/pkg/compiler/lib/src/io/position_information.dart
@@ -18,6 +18,8 @@ import '../tree/tree.dart' show
     Node,
     Send;
 
+import 'code_output.dart' show
+    CodeBuffer;
 import 'source_file.dart';
 import 'source_information.dart';
 
@@ -102,7 +104,9 @@ class PositionSourceInformation extends SourceInformation {
 
 class PositionSourceInformationStrategy
     implements JavaScriptSourceInformationStrategy {
-  const PositionSourceInformationStrategy();
+  Coverage coverage;

[Siggi Cherem (dart-lang), 2016/01/21 18:18:34]

revert?

I couldn't find where you create a PositionSourceInformationStrategy that
records coverage. It appears that you only do the coverage processing later in
the diff tool, but not in the main compiler pipepine. 

Is the idea to add it in the future/make part of dump-info? For now I'm inclined
to leave it out, but I guess I might be missing something.

[Johnni Winther, 2016/01/22 15:12:38]

Reverted

+
+  PositionSourceInformationStrategy([this.coverage]);
 
   @override
   SourceInformationBuilder createBuilderForContext(AstElement element) {
@@ -111,10 +115,39 @@ class PositionSourceInformationStrategy
 
   @override
   SourceInformationProcessor createProcessor(SourceMapper mapper) {
-    return new PositionSourceInformationProcessor(mapper);
+    return new PositionSourceInformationProcessor(mapper, coverage);
+  }
+
+  void onComplete() {
+    if (coverage != null) {
+      coverage.collapse();
+    }
+  }
+
+  @override
+  SourceInformation buildPreambleMarker() {
+    return const PreambleMarker();
   }
 }
 
+/// Marker used to tag the root nodes of non-preamble code.

[sra1, 2016/01/21 15:52:13]

non-preamble -> preamble?

[Johnni Winther, 2016/01/22 15:12:38]

Non-preamble! In output like 

...
var dart = [["_interceptors", "dart:_interceptors",, J, {
    "^": "",
    getInterceptor: function(object) {
      return void 0;
    },
...

The preamble marker is on the Fun node for 'getInterceptor' to marker that it is
a root node of code that we want to source map.

Changed the named to SourceMappedMarker.

+///
+/// 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 PreambleMarker extends SourceInformation {
+  const PreambleMarker();
+
+  @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;
@@ -225,10 +258,31 @@ class CodePosition {
   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();
 
@@ -261,198 +315,769 @@ enum CodePositionKind {
 
 /// 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;
+class PositionSourceInformationProcessor implements SourceInformationProcessor {
+  final CodePositionRecorder codePositionRecorder = new CodePositionRecorder();
+  CodePositionMap codePositionMap;
+  List<TraceListener> traceListeners;
 
-  PositionSourceInformationProcessor(this.sourceMapper);
+  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) {
-    node.accept(this);
+  void process(js.Node node, CodeBuffer codeBuffer) {
+    new JavaScriptTracer(codePositionMap, traceListeners).apply(node);
   }
 
-  void visitChildren(js.Node node) {
-    node.visitChildren(this);
+  @override
+  void onPositions(js.Node node,
+                   int startPosition,
+                   int endPosition,
+                   int closingPosition) {
+    codePositionRecorder.registerPositions(
+        node, startPosition, endPosition, closingPosition);
   }
+}
+
+/// [TraceListener] that register [SourceLocation]s with a [SourceMapper].
+class PositionTraceListener extends TraceListener {
+  final SourceMapper sourceMapper;
+
+  PositionTraceListener(this.sourceMapper);
 
-  CodePosition getCodePosition(js.Node node) {
-    return codePositions[node];
+  @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.CLOSING;
+        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:

[Siggi Cherem (dart-lang), 2016/01/21 18:18:34]

nit: replace the last 12 lines with 'default: ' ?

Or switch to an if-then-else? E.g:

   sourcePositionKind = (kind == StepKind.FUN)
       ? SourcePositionKind.CLOSING
       : (kind == StepKind.CALL)
           ? CallPosition.getSemantic...(node).sourcePositionKind
           : SourcePositionKind.START;

Or is the idea that this will diverge more in the future?

[Johnni Winther, 2016/01/22 15:12:38]

I avoid using default to get the best case coverage analysis (in the face of
enums)

+        break;
+    }
+    int codeLocation = offset.codeOffset;
+    SourceLocation sourceLocation;

[Siggi Cherem (dart-lang), 2016/01/21 18:18:34]

optional: make this a helper `getLocation` method similar to how you added
`getPosition`?

[Johnni Winther, 2016/01/22 15:12:38]

Done.

+    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);
+    }
   }
+}
 
-  /// 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;
+/// 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 (codeLocation != null && sourceLocation != null) {
-        sourceMapper.register(node, codeLocation, sourceLocation);
+      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.CLOSING);
       }
+    } 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.CLOSING);
+    } 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 statement containing the step.

[Siggi Cherem (dart-lang), 2016/01/21 18:18:34]

the statement => the enclosing statement?

(I assume this will be the same offset for any subexpression inside a single
statement?)

[Johnni Winther, 2016/01/22 15:12:38]

Yes. Comment updated.

+  final int statementOffset;
+
+  /// The left-to-right offset of the step.

[Siggi Cherem (dart-lang), 2016/01/21 18:18:34]

can you clarify? Not sure what left-to-right means. Offset from the beginning of
the statement? Offset from the beginning of a line?

[Johnni Winther, 2016/01/22 15:12:38]

Comments updated and examples added.

+  final int leftToRightOffset;
+
+  /// The subexpression offset of the step.

[Siggi Cherem (dart-lang), 2016/01/21 18:18:34]

is this relative to the beginning of the file or to the statementOffset?

[Johnni Winther, 2016/01/22 15:12:38]

Ditto.

+  final int codeOffset;
+
+  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) {
-    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);
+    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);
+      }
     }
-    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);
+    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);
     }
+  }
 
-    visitChildren(node);
+  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) {
-    visitChildren(node);
+    statementOffset = getSyntaxOffset(node);
+    visitSubexpression(
+        node, node.expression, statementOffset,
+        StepKind.EXPRESSION_STATEMENT);
+    statementOffset = null;
+    leftToRightOffset = null;
   }
 
   @override
-  visitBinary(js.Binary node) {
-    visitChildren(node);
+  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;

[Siggi Cherem (dart-lang), 2016/01/21 18:18:34]

I was expecting this to be restored to oldPosition instead.

I guess it might not matter, because on nested calls you reset the oldPosition
after visiting the arguments?

[Johnni Winther, 2016/01/22 15:12:38]

The [offsetPosition] set be parent nodes should only be used by the first child
node, cf. the (maybe to unclear) comment on [offsetPosition].

+  }
+
+  @override
+  visitNew(js.New node) {
+    visit(node.target);
+    visitList(node.arguments);
+    notifyStep(
+        node, getOffsetForNode(node, getSyntaxOffset(node)), StepKind.NEW);

[Siggi Cherem (dart-lang), 2016/01/21 18:18:34]

why for `new` don't you need the same logic as for `call`? Is it because `new`
is always the starting term?

[Johnni Winther, 2016/01/22 15:12:38]

Yes.

+    steps.add(node);
+    offsetPosition = null;
   }
 
   @override
   visitAccess(js.PropertyAccess node) {
-    visitChildren(node);
+    visit(node.receiver);
+    visit(node.selector);
   }
 
   @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);
+  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 {
-        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);
+        _nodesWithoutInfoCountByType.putIfAbsent(
+            node.runtimeType, () => 0);
+        _nodesWithoutInfoCountByType[node.runtimeType]++;
       }
     }
-    visitChildren(node);
+    _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 onPositions(js.Node node,
-                   int startPosition,
-                   int endPosition,
-                   int closingPosition) {
-    codePositions.registerPositions(
-        node, startPosition, endPosition, closingPosition);
+  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;
+  }
+}