Make MountReady more reusable, reduce allocations

Test Plan:
grunt test

src/utils/CallbackQueue.js

@@ -0,0 +1,103 @@
+/**
+ * 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.
+ *
+ * @providesModule CallbackQueue
+ */
+
+"use strict";
+
+var PooledClass = require('PooledClass');
+
+var invariant = require('invariant');
+var mixInto = require('mixInto');
+
+/**
+ * A specialized pseudo-event module to help keep track of components waiting to
+ * be notified when their DOM representations are available for use.
+ *
+ * This implements `PooledClass`, so you should never need to instantiate this.
+ * Instead, use `CallbackQueue.getPooled()`.
+ *
+ * @class ReactMountReady
+ * @implements PooledClass
+ * @internal
+ */
+function CallbackQueue() {
+  this._callbacks = null;
+  this._contexts = null;
+}
+
+mixInto(CallbackQueue, {
+
+  /**
+   * Enqueues a callback to be invoked when `notifyAll` is invoked.
+   *
+   * @param {function} callback Invoked when `notifyAll` is invoked.
+   * @param {?object} context Context to call `callback` with.
+   * @internal
+   */
+  enqueue: function(callback, context) {
+    this._callbacks = this._callbacks || [];
+    this._contexts = this._contexts || [];
+    this._callbacks.push(callback);
+    this._contexts.push(context);
+  },
+
+  /**
+   * Invokes all enqueued callbacks and clears the queue. This is invoked after
+   * the DOM representation of a component has been created or updated.
+   *
+   * @internal
+   */
+  notifyAll: function() {
+    var callbacks = this._callbacks;
+    var contexts = this._contexts;
+    if (callbacks) {
+      invariant(
+        callbacks.length === contexts.length,
+        "Mismatched list of contexts in callback queue"
+      );
+      this._callbacks = null;
+      this._contexts = null;
+      for (var i = 0, l = callbacks.length; i < l; i++) {
+        callbacks[i].call(contexts[i]);
+      }
+      callbacks.length = 0;
+      contexts.length = 0;
+    }
+  },
+
+  /**
+   * Resets the internal queue.
+   *
+   * @internal
+   */
+  reset: function() {
+    this._callbacks = null;
+    this._contexts = null;
+  },
+
+  /**
+   * `PooledClass` looks for this.
+   */
+  destructor: function() {
+    this.reset();
+  }
+
+});
+
+PooledClass.addPoolingTo(CallbackQueue);
+
+module.exports = CallbackQueue;