implement null aware ops, fixes #249

lib/runtime/dart/js.js

 dart_library.library('dart/js', null, /* Imports */[
   "dart_runtime/dart",
   'dart/core',
   'dart/collection',
   'dart/_js_helper'
 ], /* Lazy imports */[
 ], function(exports, dart, core, collection, _js_helper) {
   'use strict';
   let dartx = dart.dartx;
   dart.defineLazyProperties(exports, {
@@ skipping 57 matching lines @@
       if (!(typeof property == 'string') && !dart.is(property, core.num)) {
         dart.throw(new core.ArgumentError("property is not a String or num"));
       }
       return _convertToDart(this[_jsObject][property]);
     }
     set(property, value) {
       if (!(typeof property == 'string') && !dart.is(property, core.num)) {
         dart.throw(new core.ArgumentError("property is not a String or num"));
       }
       this[_jsObject][property] = _convertToJS(value);
+      return value;
     }
     get hashCode() {
       return 0;
     }
     ['=='](other) {
       return dart.is(other, JsObject) && this[_jsObject] === dart.dload(other, _jsObject);
     }
     hasProperty(property) {
       if (!(typeof property == 'string') && !dart.is(property, core.num)) {
         dart.throw(new core.ArgumentError("property is not a String or num"));
@@ skipping 117 matching lines @@
           this[_checkIndex](index);
         }
         return dart.as(super.get(index), E);
       }
       set(index, value) {
         dart.as(value, E);
         if (dart.is(index, core.num) && index == index[dartx.toInt]()) {
           this[_checkIndex](index);
         }
         super.set(index, value);
+        return value;
       }
       get length() {
         let len = this[_jsObject].length;
         if (typeof len === "number" && len >>> 0 === len) {
           return len;
         }
         dart.throw(new core.StateError('Bad JsArray length'));
       }
       set length(length) {
         super.set('length', length);
@@ skipping 167 matching lines @@
     let __CastType0 = dart.typedef('__CastType0', () => dart.functionType(dart.dynamic, [E]));
     return __CastType0;
   });
   let __CastType0 = __CastType0$();
   // Exports:
   exports.JsObject = JsObject;
   exports.JsFunction = JsFunction;
   exports.JsArray$ = JsArray$;
   exports.JsArray = JsArray;
 });