Generate mock libraries and assert that helpers are non-null.

tests/compiler/dart2js/resolver_test.dart

Index: tests/compiler/dart2js/resolver_test.dart
diff --git a/tests/compiler/dart2js/resolver_test.dart b/tests/compiler/dart2js/resolver_test.dart
index 3107d36f66d554c0eabd7db7079e660677c183d2..de61a56ac3a12b22fb0637b319ae7d0a3fbbdb7c 100644
--- a/tests/compiler/dart2js/resolver_test.dart
+++ b/tests/compiler/dart2js/resolver_test.dart
@@ -675,7 +675,7 @@ Future resolveConstructor(
     {List expectedWarnings: const [],
      List expectedErrors: const [],
      List expectedInfos: const [],
-     String corelib: DEFAULT_CORELIB}) {
+     Map corelib}) {
   MockCompiler compiler = new MockCompiler.internal(coreSource: corelib);
   return compiler.init().then((_) {
     compiler.parseScript(script);
@@ -911,28 +911,13 @@ Future testInitializers() {
     },
     () {
       String script = "";
-      final String CORELIB_WITH_INVALID_OBJECT =
-          '''print(var obj) {}
-             class int {}
-             class double {}
-             class bool {}
-             class String {}
-             class num {}
-             class Function {}
-             class List<E> {}
-             class Map {}
-             class Closure {}
-             class Null {}
-             class StackTrace {}
-             class Dynamic_ {}
-             class Type {}
-             class Object { Object() : super(); }
-             const proxy = 0;''';
+      final INVALID_OBJECT =
+          const { 'Object': 'class Object { Object() : super(); }' };
       return resolveConstructor(script,
           "Object o = new Object();", "Object", "", 1,
           expectedWarnings: [],
           expectedErrors: [MessageKind.SUPER_INITIALIZER_IN_OBJECT],
-          corelib: CORELIB_WITH_INVALID_OBJECT);
+          corelib: INVALID_OBJECT);
     },
   ], (f) => f());
 }