Make unit testing of the compiler work with the new isolate helper library.

tests/compiler/dart2js/mock_compiler.dart

Index: tests/compiler/dart2js/mock_compiler.dart
===================================================================
--- tests/compiler/dart2js/mock_compiler.dart	(revision 16156)
+++ tests/compiler/dart2js/mock_compiler.dart	(working copy)
@@ -89,6 +89,9 @@
   class Dynamic_ {}
   bool identical(Object a, Object b) {}''';
 
+const String DEFAULT_ISOLATE_HELPERLIB = r'''
+  class _WorkerBase {}''';
+
 class MockCompiler extends Compiler {
   List<WarningMessage> warnings;
   List<WarningMessage> errors;
@@ -98,6 +101,7 @@
   MockCompiler({String coreSource: DEFAULT_CORELIB,
                 String helperSource: DEFAULT_HELPERLIB,
                 String interceptorsSource: DEFAULT_INTERCEPTORSLIB,
+                String isolateHelperSource: DEFAULT_ISOLATE_HELPERLIB,
                 bool enableTypeAssertions: false,
                 bool enableMinification: false,
                 bool enableConcreteTypeInference: false,
@@ -117,6 +121,7 @@
 
     assertMethod = jsHelperLibrary.find(buildSourceString('assert'));
     interceptorsLibrary = createLibrary("interceptors", interceptorsSource);
+    isolateHelperLibrary = createLibrary("isolate_helper", isolateHelperSource);
 
     mainApp = mockLibrary(this, "");
     initializeSpecialClasses();