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 Label label;
   /// Mixed list of [Statement] and [Block].
   /// A [Block] represents a synthetic goto statement.
   final List statements = [];
   final List<Block> predecessors = <Block>[];
   final List<Block> successors = <Block>[];
 
   String get name => 'B$index';
 
   Block([this.label]);
 
@@ skipping 192 matching lines @@
     add("$bci $uses $name $contents <|@\n");
   }
 
   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");
   }
 
   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");
   }
 
   visitWhileTrue(WhileTrue node) {
@@ skipping 140 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;
   }
 }