implement null aware ops, fixes #249

lib/runtime/dart/core.js

 dart_library.library('dart/core', null, /* Imports */[
   "dart_runtime/dart"
 ], /* Lazy imports */[
   'dart/_js_helper',
   'dart/_internal',
   'dart/collection',
   'dart/_interceptors',
   'dart/convert'
 ], function(exports, dart, _js_helper, _internal, collection, _interceptors, convert) {
   'use strict';
@@ skipping 1161 matching lines @@
         return values == null ? null : dart.as(_js_helper.Primitives.getProperty(values, this[_getKey]()), T);
       }
       set(object, value) {
         dart.as(value, T);
         let values = _js_helper.Primitives.getProperty(object, Expando$()._EXPANDO_PROPERTY_NAME);
         if (values == null) {
           values = new Object();
           _js_helper.Primitives.setProperty(object, Expando$()._EXPANDO_PROPERTY_NAME, values);
         }
         _js_helper.Primitives.setProperty(values, this[_getKey](), value);
+        return value;
       }
       [_getKey]() {
         let key = dart.as(_js_helper.Primitives.getProperty(this, Expando$()._KEY_PROPERTY_NAME), String);
         if (key == null) {
           key = `expando$key$${(() => {
             let x = Expando$()._keyCount;
             Expando$()._keyCount = dart.notNull(x) + 1;
             return x;
           })()}`;
           _js_helper.Primitives.setProperty(this, Expando$()._KEY_PROPERTY_NAME, key);
@@ skipping 2173 matching lines @@
   exports.StackTrace = StackTrace;
   exports.Stopwatch = Stopwatch;
   exports.String = String;
   exports.RuneIterator = RuneIterator;
   exports.StringBuffer = StringBuffer;
   exports.StringSink = StringSink;
   exports.Symbol = Symbol;
   exports.Type = Type;
   exports.Uri = Uri;
 });