Set up a stub pipline for using the new cps-based ir to generate js.

pkg/compiler/lib/src/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 tracer;
 
 import '../compiler.dart' as api;
 import 'dart:async' show EventSink;
 import 'ssa/ssa.dart' as ssa;
 import 'ssa/ssa_tracer.dart' show HTracer;
 import 'cps_ir/cps_ir_nodes.dart' as cps_ir;
 import 'cps_ir/cps_ir_tracer.dart' show IRTracer;
-import 'dart_backend/tree_ir_nodes.dart' as tree_ir;
-import 'dart_backend/tree_ir_tracer.dart' show TreeTracer;
+import 'tree_ir/tree_ir_nodes.dart' as tree_ir;
+import 'tree_ir/tree_ir_tracer.dart' show TreeTracer;
 import 'util/util.dart' show Indentation;
 import 'dart2jslib.dart';
 
 /**
  * If non-null, we only trace methods whose name match the regexp defined by the
  * given pattern.
  */
 const String TRACE_FILTER_PATTERN = const String.fromEnvironment("DUMP_IR");
 
 final RegExp TRACE_FILTER =
@@ skipping 85 matching lines @@
   }
 
   void add(String string) {
     output.add(string);
   }
 
   void addIndent() {
     add(_ind.indentation);
   }
 }