Support for labels in non-loop code in new dart backend

pkg/compiler/lib/src/dart_backend/tree_ir_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 tree_ir_tracer;
 
 import 'dart:async' show EventSink;
 import '../tracer.dart';
 import 'tree_ir_nodes.dart';
 
@@ skipping 321 matching lines @@
     List<String> entries = new List<String>();
     node.entries.forEach((LiteralMapEntry entry) {
       String key = visitExpression(entry.key);
       String value = visitExpression(entry.value);
       entries.add("$key: $value");
     });
     return "map [${entries.join(', ')}]";
   }
 
   String visitConstant(Constant node) {
-    return "${node.value}";
+    return "${node.value.toStructuredString()}";
   }
 
   String visitThis(This node) {
     return "this";
   }
 
   String visitReifyTypeVar(ReifyTypeVar node) {
     return "typevar [${node.typeVariable.name}]";
   }
 
@@ skipping 58 matching lines @@
       String prefix = v.element == null ? 'v' : '${v.element.name}_';
       while (name == null || _usedNames.contains(name)) {
         name = "$prefix${_counter++}";
       }
       _names[v] = name;
       _usedNames.add(name);
     }
     return name;
   }
 }