dart2dart: Preserve variable names throughout the IR.

sdk/lib/_internal/compiler/implementation/dart_backend/tree_tracer.dart

 // Copyright (c) 2014, 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.
 
 library dart_backend.tracer;
 
 import 'dart:async' show EventSink;
 import '../tracer.dart';
 import 'dart_tree.dart';
 
 class Block {
+  Label label;
   int index;
   final List<Statement> statements = <Statement>[];
   final List<Block> predecessors = <Block>[];
   final List<Block> successors = <Block>[];
 
   String get name => 'B$index';
 
+  Block([this.label]);
+
   void addEdgeTo(Block successor) {
     successors.add(successor);
     successor.predecessors.add(this);
   }
 }
 
 class BlockCollector extends Visitor {
   // Accumulate a list of blocks.  The current block is the last block in
   // the list.
   final List<Block> blocks = [new Block()..index = 0];
@@ skipping 24 matching lines @@
   visitConcatenateStrings(ConcatenateStrings node) {}
   visitLiteralList(LiteralList node) {}
   visitLiteralMap(LiteralMap node) {}
   visitInvokeConstConstructor(InvokeConstConstructor node) {}
   visitConstant(Constant node) {}
   visitConditional(Conditional node) {}
   visitLogicalOperator(LogicalOperator node) {}
   visitNot(Not node) {}
 
   visitLabeledStatement(LabeledStatement node) {
-    Block target = new Block();
+    Block target = new Block(node.label);
     breakTargets[node.label] = target;
     visitStatement(node.body);
     _addBlock(target);
     visitStatement(node.next);
   }
 
   visitAssign(Assign node) {
     _addStatement(node);
     visitStatement(node.next);
   }
@@ skipping 72 matching lines @@
       printProperty("successors", block.successors.map((b) => b.name));
       printEmptyProperty("xhandlers");
       printEmptyProperty("flags");
       tag("states", () {
         tag("locals", () {
           printProperty("size", 0);
           printProperty("method", "None");
         });
       });
       tag("HIR", () {
+        if (block.label != null) {
+          printStatement(null,
+              "Label ${block.name}, breakCount=${block.label.breakCount}");
+        }
         block.statements.forEach(visitStatement);
       });
     });
   }
 
   void printStatement(String name, String contents) {
     int bci = 0;
     int uses = 0;
     if (name == null) {
       name = 'x${statementCounter++}';
@@ skipping 14 matching lines @@
     printStatement(null, "dead-use ${node.value}");
   }
 
   visitLabeledStatement(LabeledStatement node) {
     // These do not get added to a block's list of statements.
   }
 
   visitAssign(Assign node) {
     String name = names.varName(node.variable);
     String rhs = expr(node.definition);
-    printStatement(name, "let $name = $rhs");
+    String extra = node.hasExactlyOneUse ? "[single-use]" : "";
+    printStatement(null, "assign $name = $rhs $extra");
   }
 
   visitInvokeMethod(InvokeMethod node) {
     printStatement(null, expr(node));
   }
 
   visitInvokeConstructor(InvokeConstructor node) {
     printStatement(null, expr(node));
   }
 
@@ skipping 28 matching lines @@
   visitReturn(Return node) {
     printStatement(null, "return ${expr(node.value)}");
   }
 
   visitBreak(Break node) {
     printStatement(null, "break ${collector.breakTargets[node.target].name}");
   }
 
   visitContinue(Continue node) {
     printStatement(null,
-        "continue ${collector.breakTargets[node.target].name}");
+        "continue ${collector.continueTargets[node.target].name}");
   }
 
   visitIf(If node) {
     String condition = expr(node.condition);
     String thenTarget = collector.ifTargets[node.thenStatement].name;
     String elseTarget = collector.ifTargets[node.elseStatement].name;
     printStatement(null, "if $condition then $thenTarget else $elseTarget");
   }
 
   visitWhile(While node) {
@@ skipping 147 matching lines @@
  * name to avoid clashing with a previously synthesized variable name.
  */
 class Names {
   final Map<Variable, String> _names = {};
   final Set<String> _usedNames = new Set();
   int _counter = 0;
 
   String varName(Variable v) {
     String name = _names[v];
     if (name == null) {
-      name = v.name;
-      if (v.cachedName != null) {
-        name = v.cachedName;
-      }
+      String prefix = v.element == null ? 'v' : '${v.element.name}_';
       while (name == null || _usedNames.contains(name)) {
-        name = "v${_counter++}";
+        name = "$prefix${_counter++}";
       }
       _names[v] = name;
       _usedNames.add(name);
     }
     return name;
   }
 }