Move dart2jslib parts into separate libraries.

tests/compiler/dart2js/compiler_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 "dart:async";
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
-import "package:compiler/src/dart2jslib.dart";
 import "package:compiler/src/elements/elements.dart";
+import "package:compiler/src/messages.dart";
 import "package:compiler/src/resolution/resolution.dart";
 import "package:compiler/src/util/util.dart";
 import "mock_compiler.dart";
 
 
 class CallbackMockCompiler extends MockCompiler {
   CallbackMockCompiler() : super.internal();
 
   var onError;
   var onWarning;
@@ skipping 33 matching lines @@
     compiler.setOnWarning(
         (c, n, m) => Expect.equals(foo, compiler.currentElement));
     foo.computeType(compiler);
     Expect.equals(1, compiler.warnings.length);
   });
 }
 
 main() {
   asyncTest(() => testErrorHandling());
 }