Add test for isValidDescriptor

Fixes #1164.

Test Plan: grunt test

src/core/__tests__/ReactDescriptor-test.js

@@ -0,0 +1,48 @@
+/**
+ * Copyright 2013-2014 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * @jsx React.DOM
+ * @emails react-core
+ */
+
+"use strict";
+
+var React;
+var ReactDescriptor;
+
+describe('ReactDescriptor', function() {
+  beforeEach(function() {
+    React = require('React');
+    ReactDescriptor = require('ReactDescriptor');
+  });
+
+  it('should identify valid descriptors correctly', function() {
+    var Component = React.createClass({
+      render: function() {
+        return <div />;
+      }
+    });
+
+    expect(ReactDescriptor.isValidDescriptor(<div />)).toEqual(true);
+    expect(ReactDescriptor.isValidDescriptor(<Component />)).toEqual(true);
+
+    expect(ReactDescriptor.isValidDescriptor(null)).toEqual(false);
+    expect(ReactDescriptor.isValidDescriptor(true)).toEqual(false);
+    expect(ReactDescriptor.isValidDescriptor({})).toEqual(false);
+    expect(ReactDescriptor.isValidDescriptor("string")).toEqual(false);
+    expect(ReactDescriptor.isValidDescriptor(React.DOM.div)).toEqual(false);
+    expect(ReactDescriptor.isValidDescriptor(Component)).toEqual(false);
+  });
+});