implement null aware ops, fixes #249

lib/runtime/dart/_internal.js

 dart_library.library('dart/_internal', null, /* Imports */[
   "dart_runtime/dart",
   'dart/core',
   'dart/collection'
 ], /* Lazy imports */[
   'dart/math',
   'dart/_interceptors',
   'dart/_js_primitives'
 ], function(exports, dart, core, collection, math, _interceptors, _js_primitives) {
   'use strict';
@@ skipping 1637 matching lines @@
       })
     });
     return FixedLengthListMixin;
   });
   let FixedLengthListMixin = FixedLengthListMixin$();
   let UnmodifiableListMixin$ = dart.generic(function(E) {
     class UnmodifiableListMixin extends core.Object {
       set(index, value) {
         dart.as(value, E);
         dart.throw(new core.UnsupportedError("Cannot modify an unmodifiable list"));
+        return value;
       }
       set length(newLength) {
         dart.throw(new core.UnsupportedError("Cannot change the length of an unmodifiable list"));
       }
       setAll(at, iterable) {
         dart.as(iterable, core.Iterable$(E));
         dart.throw(new core.UnsupportedError("Cannot modify an unmodifiable list"));
       }
       add(value) {
         dart.as(value, E);
@@ skipping 182 matching lines @@
         for (let i = 0; dart.notNull(i) < dart.notNull(length); i = dart.notNull(i) + 1) {
           f(i, this[_values][dartx.get](i));
           if (length != this[_values][dartx.length]) {
             dart.throw(new core.ConcurrentModificationError(this[_values]));
           }
         }
       }
       set(key, value) {
         dart.as(value, E);
         dart.throw(new core.UnsupportedError("Cannot modify an unmodifiable map"));
+        return value;
       }
       putIfAbsent(key, ifAbsent) {
         dart.as(ifAbsent, dart.functionType(E, []));
         dart.throw(new core.UnsupportedError("Cannot modify an unmodifiable map"));
       }
       remove(key) {
         dart.throw(new core.UnsupportedError("Cannot modify an unmodifiable map"));
       }
       clear() {
         dart.throw(new core.UnsupportedError("Cannot modify an unmodifiable map"));
@@ skipping 576 matching lines @@
   exports.ReversedListIterable = ReversedListIterable;
   exports.UnmodifiableListError = UnmodifiableListError;
   exports.NonGrowableListError = NonGrowableListError;
   exports.makeListFixedLength = makeListFixedLength;
   exports.Lists = Lists;
   exports.printToConsole = printToConsole;
   exports.Sort = Sort;
   exports.Symbol = Symbol;
   exports.POWERS_OF_TEN = POWERS_OF_TEN;
 });