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Closure Compiler is a JavaScript optimizing compiler. It parses your JavaScript, analyzes it, removes dead code and rewrites and minimizes what's left. It also checks syntax, variable references, and types, and warns about common JavaScript pitfalls. It is used in many of Google's JavaScript apps, including Gmail, Google Web Search, Google Maps, and Google Docs.

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/*
 * Copyright 2016 The Closure Compiler Authors.
 *
 * 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.
 */

'require es6/conformance';
'require es6/symbol';
'require es6/util/makeiterator';
'require util/polyfill';
'require es6/weakmap';

$jscomp.polyfill('WeakSet',
    /**
     * @param {*} NativeWeakSet
     * @return {*}
     * @suppress {reportUnknownTypes}
     */
    function(NativeWeakSet) {
  /**
   * Checks conformance of the existing WeakSet.
   * @return {boolean} True if the browser's implementation conforms.
   */
  function isConformant() {
    if (!NativeWeakSet || !Object.seal) return false;
    try {
      var x = Object.seal({});
      var y = Object.seal({});
      var set = new /** @type {function(new: WeakSet, !Array)} */ (
          NativeWeakSet)([x]);
      if (!set.has(x) || set.has(y)) return false;
      set.delete(x);
      set.add(y);
      return !set.has(x) && set.has(y);
    } catch (err) { // This should hopefully never happen, but let's be safe.
      return false;
    }
  }
  if ($jscomp.USE_PROXY_FOR_ES6_CONFORMANCE_CHECKS) {
    if (NativeWeakSet && $jscomp.ES6_CONFORMANCE) return NativeWeakSet;
  } else {
    if (isConformant()) return NativeWeakSet;
  }

  /**
   * @constructor
   * @extends {WeakSet}
   * @template TYPE
   * @param {!Iterator|!Array|null=} opt_iterable
   */
  var PolyfillWeakSet = function(opt_iterable) {
    /** @private @const {!WeakMap} */
    this.map_ = new WeakMap();

    if (opt_iterable) {
      $jscomp.initSymbol();
      $jscomp.initSymbolIterator();
      var iter = $jscomp.makeIterator(opt_iterable);
      var entry;
      while (!(entry = iter.next()).done) {
        var item = entry.value;
        this.add(item);
      }
    }
  };

  /** @override */
  PolyfillWeakSet.prototype.add = function(elem) {
    this.map_.set(elem, true);
    return this;
  };

  /** @override */
  PolyfillWeakSet.prototype.has = function(elem) {
    return this.map_.has(elem);
  };

  /** @override */
  PolyfillWeakSet.prototype.delete = function(elem) {
    return this.map_.delete(elem);
  };

  return PolyfillWeakSet;
}, 'es6', 'es3');