dart2dart: Preserve variable names throughout the IR.

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

Index: sdk/lib/_internal/compiler/implementation/dart_backend/tree_tracer.dart
diff --git a/sdk/lib/_internal/compiler/implementation/dart_backend/tree_tracer.dart b/sdk/lib/_internal/compiler/implementation/dart_backend/tree_tracer.dart
index 134a8bd4cc21f252a52702a669d409357570be9f..192a07e45c38df49e8006dddde548bbf355a660f 100644
--- a/sdk/lib/_internal/compiler/implementation/dart_backend/tree_tracer.dart
+++ b/sdk/lib/_internal/compiler/implementation/dart_backend/tree_tracer.dart
@@ -9,6 +9,7 @@ import '../tracer.dart';
 import 'dart_tree.dart';
 
 class Block {
+  Label label;
   int index;
   final List<Statement> statements = <Statement>[];
   final List<Block> predecessors = <Block>[];
@@ -16,6 +17,8 @@ class Block {
 
   String get name => 'B$index';
 
+  Block([this.label]);
+
   void addEdgeTo(Block successor) {
     successors.add(successor);
     successor.predecessors.add(this);
@@ -60,7 +63,7 @@ class BlockCollector extends Visitor {
   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);
@@ -153,6 +156,10 @@ class TreeTracer extends TracerUtil with Visitor {
         });
       });
       tag("HIR", () {
+        if (block.label != null) {
+          printStatement(null,
+              "Label ${block.name}, breakCount=${block.label.breakCount}");
+        }
         block.statements.forEach(visitStatement);
       });
     });
@@ -187,7 +194,8 @@ class TreeTracer extends TracerUtil with Visitor {
   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) {
@@ -236,7 +244,7 @@ class TreeTracer extends TracerUtil with Visitor {
 
   visitContinue(Continue node) {
     printStatement(null,
-        "continue ${collector.breakTargets[node.target].name}");
+        "continue ${collector.continueTargets[node.target].name}");
   }
 
   visitIf(If node) {
@@ -404,12 +412,9 @@ class Names {
   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);