implement null aware ops, fixes #249

lib/runtime/dart/_interceptors.js

 dart_library.library('dart/_interceptors', null, /* Imports */[
   "dart_runtime/dart",
   'dart/core',
   'dart/_internal',
   'dart/collection',
   'dart/math'
 ], /* Lazy imports */[
   'dart/_js_helper'
 ], function(exports, dart, core, _internal, collection, math, _js_helper) {
   'use strict';
@@ skipping 368 matching lines @@
           dart.throw(new core.RangeError.value(index));
         return dart.as(this[index], E);
       }
       [dartx.set](index, value) {
         dart.as(value, E);
         if (!(typeof index == 'number'))
           dart.throw(new core.ArgumentError(index));
         if (dart.notNull(index) >= dart.notNull(this[dartx.length]) || dart.notNull(index) < 0)
           dart.throw(new core.RangeError.value(index));
         this[index] = value;
+        return value;
       }
       [dartx.asMap]() {
         return new (_internal.IterableMixinWorkaround$(E))().asMapList(this);
       }
     }
     dart.setBaseClass(JSArray, dart.global.Array);
     JSArray[dart.implements] = () => [core.List$(E), JSIndexable];
     dart.setSignature(JSArray, {
       constructors: () => ({
         JSArray: [JSArray$(E), []],
@@ skipping 1192 matching lines @@
   exports.JSString = JSString;
   exports.getInterceptor = getInterceptor;
   exports.JSBool = JSBool;
   exports.JSIndexable = JSIndexable;
   exports.JSMutableIndexable = JSMutableIndexable;
   exports.JSObject = JSObject;
   exports.JavaScriptObject = JavaScriptObject;
   exports.PlainJavaScriptObject = PlainJavaScriptObject;
   exports.UnknownJavaScriptObject = UnknownJavaScriptObject;
 });