Don't hide interceptors in mirrors.

tests/utils/dummy_compiler_test.dart

Index: tests/utils/dummy_compiler_test.dart
diff --git a/tests/utils/dummy_compiler_test.dart b/tests/utils/dummy_compiler_test.dart
index f305e9cc8eb0d8c23373cf5bb41e371ce483003f..17436151a61e9a36285b6acd01523850b310f96f 100644
--- a/tests/utils/dummy_compiler_test.dart
+++ b/tests/utils/dummy_compiler_test.dart
@@ -22,6 +22,7 @@ Future<String> provider(Uri uri) {
 library core;
 class Object {
   Object();
+  // Note: JSNull below must reimplement all members.
   operator==(other) {}
   get hashCode => throw 'Object.hashCode not implemented.';
 }
@@ -84,7 +85,10 @@ class JSUInt31 {}
 class JSUInt32 {}
 class JSDouble {}
 class JSNumber {}
-class JSNull {}
+class JSNull {
+  bool operator ==(other) => identical(null, other);
+  int get hashCode => 0;
+}
 class JSBool {}
 getInterceptor(o){}
 getDispatchProperty(o) {}