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

tests/compiler/dart2js/erroneous_element_test.dart

 // Copyright (c) 2012, 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/elements/elements.dart';
+import 'package:compiler/implementation/elements/elements.dart';
 import 'parser_helper.dart';
 
-import 'package:compiler/src/elements/modelx.dart'
+import 'package:compiler/implementation/elements/modelx.dart'
     show ErroneousElementX;
 
-import 'package:compiler/src/dart2jslib.dart'
+import 'package:compiler/implementation/dart2jslib.dart'
     show MessageKind;
 
 void main() {
   ErroneousElement e = new ErroneousElementX(MessageKind.GENERIC,
                                              {'text': 'error'},
                                              'foo', null);
   Expect.stringEquals('<foo: error>', '$e');
 }