Add proper API for creating private symbols wrt a library.

tests/lib/mirrors/intercepted_class_test.dart

Index: tests/lib/mirrors/intercepted_class_test.dart
diff --git a/tests/lib/mirrors/intercepted_class_test.dart b/tests/lib/mirrors/intercepted_class_test.dart
index 2fde1b71b9e128307f516d205241fb51313cf697..0eb227d33a862fac8ed48060c097a1d563a88b03 100644
--- a/tests/lib/mirrors/intercepted_class_test.dart
+++ b/tests/lib/mirrors/intercepted_class_test.dart
@@ -7,17 +7,28 @@
 library test.intercepted_class_test;
 
 import 'dart:mirrors';
+import 'package:expect/expect.dart';
 
 import 'stringify.dart' show stringify, expect;
 
 checkClassMirror(ClassMirror cls, String name) {
-  expect('s($name)', cls.simpleName);
+  bool foundInterface = false;
+  for(ClassMirror cm = cls; cm != null; cm = cm.superclass) {
+    if (MirrorSystem.getName(cm.simpleName) == name) foundInterface = true;
+    cm.superinterfaces.forEach((intf) {
+      if (MirrorSystem.getName(intf.simpleName) == name) foundInterface = true;
+    });
+  }
+  Expect.isTrue(foundInterface, '$cls should implement $name');
+
   var variables = new Map();
-  cls.variables.forEach((Symbol key, VariableMirror value) {
-    if (!value.isStatic && !value.isPrivate) {
-      variables[key] = value;
-    }
-  });
+  for(ClassMirror cm = cls; cm != null; cm = cm.superclass) {
+    cm.variables.forEach((Symbol key, VariableMirror value) {
+      if (!value.isStatic && !value.isPrivate) {
+        variables[key] = value;
+      }
+    });
+  }
   expect('{}', variables);
 }