Move dart2jslib parts into separate libraries.

tests/compiler/dart2js/issue13354_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.
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 import 'compiler_helper.dart';
-import 'parser_helper.dart';
 import 'type_mask_test_helper.dart';
 
 const String TEST = """
 bar() => 42;
 baz() => bar;
 
 class A {
   foo() => 42;
 }
 
@@ skipping 29 matching lines @@
           simplify(typesInferrer.getReturnTypeOfElement(element), compiler));
     }
 
     checkReturn('bar', typesTask.uint31Type);
     checkReturn('baz', typesTask.functionType);
 
     checkReturnInClass('A', 'foo', typesTask.uint31Type);
     checkReturnInClass('B', 'foo', typesTask.functionType);
   }));
 }