Revert "Move dart2js from sdk/lib/_internal/compiler to pkg/compiler"

tests/compiler/dart2js/stats_test.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.
 
 import 'package:expect/expect.dart';
-import 'package:compiler/src/helpers/helpers.dart';
+import 'package:compiler/implementation/helpers/helpers.dart';
 
 class CollectingOutput implements StatsOutput {
   final StringBuffer sb = new StringBuffer();
 
   void println(String text) {
     sb.write('$text\n');
   }
 
   String toString() => sb.toString();
 }
@@ skipping 66 matching lines @@
 }
 
 void test(f(Stats stats), expectedDump) {
   CollectingOutput output = new CollectingOutput();
   Stats stats = new ActiveStats(new ConsolePrinter(output: output));
   f(stats);
   stats.dumpStats();
   print(output.toString());
   Expect.equals(expectedDump, output.toString());
 }