metalet with postfix improvement

lib/runtime/dart_runtime.js

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 var dart, _js_helper;
 (function (dart) {
   'use strict';
 
   let defineProperty = Object.defineProperty;
   let getOwnPropertyDescriptor = Object.getOwnPropertyDescriptor;
@@ skipping 284 matching lines @@
   dart.equals = equals;
 
   /** Checks that `x` is not null or undefined. */
   function notNull(x) {
     if (x == null) throw 'expected not-null value';
     return x;
   }
   dart.notNull = notNull;
 
   /**
+   * A null-checking property wrapper.
+   *
+   * Used when we need an assignable expression:
+   *
+   *     obj.foo++; // normal variant
+   *     prop(obj, 'foo').notNull++; // checking variant
+   *
+   * This checks on read, not on write.
+   */
+  function prop(obj, prop) {
+    return {
+      get notNull() { return notNull(obj[prop]); },
+      set notNull(x) { obj[prop] = x; }
+    };
+  }
+  dart.prop = prop;
+
+  /** A null checking get/set index wrapper. */
+  function index(obj, index) {
+    return {
+      get notNull() { return notNull(obj[core.$get](index)); },
+      set notNull(x) { obj[core.$set](index, x); }
+    };
+  }
+  dart.index = index;
+
+  /**
    * Defines a lazy property.
    * After initial get or set, it will replace itself with a value property.
    */
   // TODO(jmesserly): is this the best implementation for JS engines?
   // TODO(jmesserly): reusing descriptor objects has been shown to improve
   // performance in other projects (e.g. webcomponents.js ShadowDOM polyfill).
   function defineLazyProperty(to, name, desc) {
     let init = desc.get;
     let writable = !!desc.set;
     function lazySetter(value) {
@@ skipping 194 matching lines @@
   // of some sort, assuming we keep around `dynamic` at runtime.
   dart.dynamic = { toString() { return 'dynamic'; } };
 
   dart.JsSymbol = Symbol;
 
   // TODO(jmesserly): hack to bootstrap the SDK
   _js_helper = _js_helper || {};
   _js_helper.checkNum = notNull;
 
 })(dart || (dart = {}));