Make sure that libraries loaded from dart:core gets dart:core implicitly loaded.

tests/compiler/dart2js/compiler_helper.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.
 // Test constant folding.
 
 #library("compiler_helper");
 
 #import("dart:uri");
 #import("../../../lib/compiler/implementation/leg.dart", prefix: "leg");
 #import("../../../lib/compiler/implementation/js_backend/js_backend.dart", prefix: "js");
@@ skipping 18 matching lines @@
 MockCompiler compilerFor(String code, Uri uri) {
   MockCompiler compiler = new MockCompiler();
   compiler.sourceFiles[uri.toString()] = new SourceFile(uri.toString(), code);
   return compiler;
 }
 
 String compileAll(String code) {
   Uri uri = new Uri.fromComponents(scheme: 'source');
   MockCompiler compiler = compilerFor(code, uri);
   compiler.runCompiler(uri);
+  Expect.isFalse(compiler.compilationFailed,
+                 'Unexpected compilation error');
   return compiler.assembledCode;
 }
 
 lego.Element findElement(var compiler, String name) {
   var element = compiler.mainApp.find(buildSourceString(name));
   Expect.isNotNull(element, 'Could not locate $name.');
   return element;
 }
 
 String anyIdentifier = "[a-zA-Z][a-zA-Z0-9]*";
@@ skipping 18 matching lines @@
   String generated = compile(code, entry);
   Expect.isTrue(regexp.hasMatch(generated),
                 '"$generated" does not match /$regexp/');
 }
 
 void compileAndDoNotMatch(String code, String entry, RegExp regexp) {
   String generated = compile(code, entry);
   Expect.isFalse(regexp.hasMatch(generated),
                  '"$generated" has a match in /$regexp/');
 }