Update to use one less proxy per element

lib/src/js/polymer_array_methods.html

-<link rel="import" href="polymer_base_dart.html">
-<script>
-  (function() {
-    var _originalPush = Polymer.Base.push;
-    Polymer.Base.push = function(path) {
-      var value = _originalPush.apply(this, arguments);
-      var dartList = this.get(path).__dartClass__;
-      if (dartList == null) return value;
-      Polymer.Dart.push(dartList, Array.prototype.splice.call(arguments, 1));
-      return value;
-    };
-
-    Polymer.Base._originalPop = Polymer.Base.pop;
-    Polymer.Base.pop = function(path) {
-      var value = this._originalPop(path);
-      var dartList = this.get(path).__dartClass__;
-      if (dartList == null) return value;
-      Polymer.Dart.pop(dartList);
-      return value;
-    };
-
-    Polymer.Base._originalShift = Polymer.Base.shift;
-    Polymer.Base.shift = function(path) {
-      var value = this._originalShift(path);
-      var dartList = this.get(path).__dartClass__;
-      if (dartList == null) return value;
-      Polymer.Dart.shift(dartList);
-      return value;
-    };
-
-    var _originalUnshift = Polymer.Base.unshift;
-    Polymer.Base.unshift = function(path) {
-      var value = _originalUnshift.apply(this, arguments);
-      var dartList = this.get(path).__dartClass__;
-      if (dartList == null) return value;
-      Polymer.Dart.unshift(dartList, Array.prototype.splice.call(arguments, 1));
-      return value;
-    };
-
-    var _originalSplice = Polymer.Base.splice;
-    Polymer.Base.splice = function(path, index, removeCount) {
-      var value = _originalSplice.apply(this, arguments);
-      var dartList = this.get(path).__dartClass__;
-      if (dartList == null) return value;
-      Polymer.Dart.splice(
-          dartList, index, removeCount,
-          Array.prototype.splice.call(arguments, 3));
-      return value;
-    };
-  })();
-</script>