Add a command line option to use the new CPS based backend.

pkg/compiler/lib/src/dart_backend/backend_ast_to_frontend_ast.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_tree_printer;
 
 import '../constants/values.dart' as values;
 import '../dart_types.dart' as types;
 import '../dart2jslib.dart' as dart2js;
 import '../elements/elements.dart' as elements;
 import '../tree/tree.dart' as tree;
 import '../scanner/scannerlib.dart';
 import '../util/util.dart';
 import 'backend_ast_nodes.dart';
 import 'backend_ast_emitter.dart' show TypeGenerator;
 
 /// Translates the backend AST to Dart frontend AST.
 tree.Node emit(dart2js.TreeElementMapping treeElements,
                RootNode root) {
   return new TreePrinter(treeElements).makeDefinition(root);
 }
 
 /// If true, the unparser will insert a coment in front of every function
 /// it emits. This helps indicate which functions were translated by the new
 /// backend.
 bool INSERT_NEW_BACKEND_COMMENT =
-    const bool.fromEnvironment('USE_NEW_BACKEND', defaultValue: false);
+    const bool.fromEnvironment("INSERT_NEW_BACKEND_COMMENT");
 
 /// Converts backend ASTs to frontend ASTs.
 class TreePrinter {
   dart2js.TreeElementMapping treeElements;
 
   TreePrinter([this.treeElements]);
 
   tree.Node makeDefinition(RootNode node) {
     if (node is FieldDefinition) {
       tree.Node definition;
@@ skipping 1294 matching lines @@
             printStringChunk(chunk.previous),
             node);
       } else {
         return node;
       }
     }
     return printStringChunk(output.chunk);
   }
 
 }