Move dart2jslib parts into separate libraries.

tests/compiler/dart2js/constant_folding_codeUnitAt_test.dart

 // Copyright (c) 2014, 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 constant folding on numbers.
 
 import 'dart:async';
-import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
 import 'compiler_helper.dart';
 
 const String TEST_1 = """
 foo() {
   var a = 'Hello';
   var b = 0;
   return a.codeUnitAt(b);
 }
 """;
@@ skipping 18 matching lines @@
 
 main() {
   asyncTest(() => Future.wait([
     compileAndMatch(TEST_1, 'foo', new RegExp(r'return 72')),
     compileAndDoNotMatch(TEST_1, 'foo', new RegExp(r'Hello')),
 
     compileAndMatch(TEST_2, 'foo', new RegExp(r'Hello')),
     compileAndMatch(TEST_3, 'foo', new RegExp(r'Hello')),
   ]));
 }