Move dart2jslib parts into separate libraries.

tests/compiler/dart2js/erroneous_element_test.dart

Index: tests/compiler/dart2js/erroneous_element_test.dart
diff --git a/tests/compiler/dart2js/erroneous_element_test.dart b/tests/compiler/dart2js/erroneous_element_test.dart
index 154ff0a479b32eac2c640c4b3960be3450150211..3bc0e43cdb185c7f485dd1163858b6fc84ea2003 100644
--- a/tests/compiler/dart2js/erroneous_element_test.dart
+++ b/tests/compiler/dart2js/erroneous_element_test.dart
@@ -2,14 +2,13 @@
 // 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:expect/expect.dart';
 import 'package:compiler/src/elements/elements.dart';
-import 'parser_helper.dart';
 
 import 'package:compiler/src/elements/modelx.dart'
     show ErroneousElementX;
 
-import 'package:compiler/src/dart2jslib.dart'
+import 'package:compiler/src/messages.dart'
     show MessageKind;
 
 void main() {