general cleanup (#256)

This patch:
- fixes multimap implementation to work properly in node
- moves ESTreeWalker from third-party into utils/doclint. ESTreeWalker
  license is compliant with Apache2.0.

lib/Multimap.js

@@ -94,13 +94,6 @@ class Multimap {
     this._map.delete(key);
   }
 
-  /**
-   * @return {!Array<K>}
-   */
-  keysArray() {
-    return this._map.keysArray();
-  }
-
   /**
    * @param {K} key
    * @return {?V} value
@@ -124,9 +117,8 @@ class Multimap {
    */
   valuesArray() {
     let result = [];
-    let keys = this.keysArray();
-    for (let i = 0; i < keys.length; ++i)
-      result.pushAll(this.get(keys[i]).valuesArray());
+    for (let key of this._map.keys())
+      result.push(...Array.from(this._map.get(key).values()));
     return result;
   }