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

tests/compiler/dart2js/backend_dart/dart_new_backend_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 '../compiler_helper.dart';
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
-import 'package:compiler/src/dart2jslib.dart';
-import 'package:compiler/src/dart_backend/dart_backend.dart';
-import 'package:compiler/src/dart_backend/backend_ast_to_frontend_ast.dart'
+import 'package:compiler/implementation/dart2jslib.dart';
+import 'package:compiler/implementation/dart_backend/dart_backend.dart';
+import 'package:compiler/implementation/dart_backend/backend_ast_to_frontend_ast.dart'
     show INSERT_NEW_BACKEND_COMMENT;
 
 const String TestStaticField = """
 class Foo {
   static int x = 1;
 }
 main() {
   print(Foo.x);
 }
 """;
@@ skipping 31 matching lines @@
   });
 }
 
 main() {
   INSERT_NEW_BACKEND_COMMENT = true;
 
   asyncTest(() => Future.forEach([
     () => test(TestStaticField, 'static fields'),
   ], (f) => f()));
 }