Load builtin helper libraries on demand.

tests/compiler/dart2js/library_load_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 'mock_compiler.dart';
+
+class ScanMockCompiler extends MockCompiler {
+  ScanMockCompiler() {
+    isolateHelperLibrary = null;
+    foreignLibrary = null;
+  }
+
+  LibraryElement scanBuiltinLibrary(String filename) {
+    return createLibrary(filename, "main(){}");
+  }
+}
+
+void main() {
+  Compiler compiler = new ScanMockCompiler();
+  Expect.equals(null, compiler.isolateHelperLibrary);
+  Expect.equals(null, compiler.foreignLibrary);
+  compiler.onLibraryLoaded(mockLibrary(compiler, "mock"),
+                            new Uri(scheme: 'dart', path: '_isolate_helper'));
+  Expect.isTrue(compiler.isolateHelperLibrary != null);
+  compiler.onLibraryLoaded(mockLibrary(compiler, "mock"),
+                            new Uri(scheme: 'dart', path: '_foreign_helper'));
+  Expect.isTrue(compiler.isolateHelperLibrary != null);
+  Expect.equals(new Uri(scheme: 'dart', path: '_isolate_helper'),
+                 compiler.isolateHelperLibrary.canonicalUri);
+  Expect.isTrue(compiler.foreignLibrary != null);
+  Expect.equals(new Uri(scheme: 'dart', path: '_foreign_helper'),
+                 compiler.foreignLibrary.canonicalUri);
+}