Support using a mock SDK with the resolver in code-transformers, and switch

pkg/code_transformers/test/resolver_test.dart

Index: pkg/code_transformers/test/resolver_test.dart
diff --git a/pkg/code_transformers/test/resolver_test.dart b/pkg/code_transformers/test/resolver_test.dart
index 3ab00e741e5151b5327e77e1d487c9876c59e2e9..5807fd718eca15b1067d5a1700bbd15d8d016b0d 100644
--- a/pkg/code_transformers/test/resolver_test.dart
+++ b/pkg/code_transformers/test/resolver_test.dart
@@ -14,9 +14,21 @@ import 'package:unittest/unittest.dart';
 
 main() {
   useCompactVMConfiguration();
-  var entryPoint = new AssetId('a', 'web/main.dart');
-  var resolvers = new Resolvers(testingDartSdkDirectory);
+  group('mock sdk', () {
+    resolverTests(new Resolvers.fromMock({
+      'dart:core': 'library dart.core;\nclass Object {}',
+      'dart:async': 'library dart.async;',
+      'dart:html': 'library dart.html;',
+    }));
+  });
+
+  group('real sdk', () {
+    resolverTests(new Resolvers(testingDartSdkDirectory));
+  });
+}
 
+resolverTests(Resolvers resolvers) {
+  var entryPoint = new AssetId('a', 'web/main.dart');
   Future validateResolver({Map<String, String> inputs, validator(Resolver),
       List<String> messages: const[]}) {
     return applyTransformers(