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

tests/compiler/dart2js/unneeded_part_js_test.dart

 // Copyright (c) 2013, 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.
 
 // Test that no parts are emitted when deferred loading isn't used.
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 import 'memory_source_file_helper.dart';
 
-import 'package:compiler/src/dart2jslib.dart'
+import 'package:compiler/implementation/dart2jslib.dart'
        show NullSink;
 
 import 'package:compiler/compiler.dart'
        show Diagnostic;
 
 import 'dart:async';
 
 main() {
   Uri script = currentDirectory.resolveUri(Platform.script);
   Uri libraryRoot = script.resolve('../../../sdk/');
@@ skipping 36 matching lines @@
 
 main() {
   var x = fisk;
   if (new DateTime.now().millisecondsSinceEpoch == 42) {
     x = new Greeting(\"I\'m confused\");
   }
   print(x.message);
 }
 """,
 };