Add the most useful Apache Commons utils

compiler/java/com/google/dart/compiler/util/apache/mutable/Mutable.java

Index: compiler/java/com/google/dart/compiler/util/apache/mutable/Mutable.java
diff --git a/compiler/java/com/google/dart/compiler/util/apache/mutable/Mutable.java b/compiler/java/com/google/dart/compiler/util/apache/mutable/Mutable.java
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+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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.
+ */
+
+package com.google.dart.compiler.util.apache.mutable;
+
+/**
+ * Provides mutable access to a value.
+ * <p>
+ * <code>Mutable</code> is used as a generic interface to the implementations in this package.
+ * <p>
+ * A typical use case would be to enable a primitive or string to be passed to a method and allow that method to
+ * effectively change the value of the primitive/string. Another use case is to store a frequently changing primitive in
+ * a collection (for example a total in a map) without needing to create new Integer/Long wrapper objects.
+ * <p>
+ * NOTICE: This file is modified copy of its original Apache library.
+ * It was moved to the different package, and changed to reduce number of dependencies.
+ * 
+ * @since 2.1
+ * @param <T> the type to set and get 
+ * @version $Id: Mutable.java 1153213 2011-08-02 17:35:39Z ggregory $
+ */
+public interface Mutable<T> {
+
+    /**
+     * Gets the value of this mutable.
+     * 
+     * @return the stored value
+     */
+    T getValue();
+
+    /**
+     * Sets the value of this mutable.
+     * 
+     * @param value
+     *            the value to store
+     * @throws NullPointerException
+     *             if the object is null and null is invalid
+     * @throws ClassCastException
+     *             if the type is invalid
+     */
+    void setValue(T value);
+
+}