implement null aware ops, fixes #249

lib/runtime/dart/convert.js

 dart_library.library('dart/convert', null, /* Imports */[
   "dart_runtime/dart",
   'dart/core',
   'dart/async',
   'dart/typed_data',
   'dart/_internal',
   'dart/collection'
 ], /* Lazy imports */[
 ], function(exports, dart, core, async, typed_data, _internal, collection) {
   'use strict';
@@ skipping 2647 matching lines @@
       } else if (dart.notNull(this.containsKey(key))) {
         let processed = this[_processed];
         _JsonMap._setProperty(processed, dart.as(key, core.String), value);
         let original = this[_original];
         if (!dart.notNull(core.identical(original, processed))) {
           _JsonMap._setProperty(original, dart.as(key, core.String), null);
         }
       } else {
         this[_upgrade]().set(key, value);
       }
+      return value;
     }
     addAll(other) {
       other.forEach(dart.fn(((key, value) => {
         this.set(key, value);
       }).bind(this)));
     }
     containsValue(value) {
       if (dart.notNull(this[_isUpgraded]))
         return this[_upgradedMap].containsValue(value);
       let keys = this[_computeKeys]();
@@ skipping 216 matching lines @@
   exports.LineSplitter = LineSplitter;
   exports.StringConversionSink = StringConversionSink;
   exports.ClosableStringSink = ClosableStringSink;
   exports.UNICODE_REPLACEMENT_CHARACTER_RUNE = UNICODE_REPLACEMENT_CHARACTER_RUNE;
   exports.UNICODE_BOM_CHARACTER_RUNE = UNICODE_BOM_CHARACTER_RUNE;
   exports.Utf8Codec = Utf8Codec;
   exports.UTF8 = UTF8;
   exports.Utf8Encoder = Utf8Encoder;
   exports.Utf8Decoder = Utf8Decoder;
 });