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

tests/compiler/dart2js/backend_dart/test_helper.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_backend.test_helper;
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
-import 'package:compiler/implementation/dart2jslib.dart';
+import 'package:compiler/src/dart2jslib.dart';
 import '../../../../pkg/analyzer2dart/test/test_helper.dart';
 import '../compiler_helper.dart';
 
 /// Compiles the given dart code (which must include a 'main' function) and
 /// returns the compiler.
 Future<Compiler> compilerFor(String code) {
   MockCompiler compiler = new MockCompiler.internal(
       emitJavaScript: false,
       enableMinification: false);
   compiler.diagnosticHandler = createHandler(compiler, code);
@@ skipping 14 matching lines @@
     compiler.irBuilder.buildNodes(useNewBackend: true);
 
     return compiler;
   });
 }
 
 /// Test group using async_helper.
 asyncTester(Group group, RunTest runTest) {
   asyncTest(() => Future.forEach(group.results, runTest));
 }