metalet with postfix improvement

lib/runtime/dart_runtime.js

Index: lib/runtime/dart_runtime.js
diff --git a/lib/runtime/dart_runtime.js b/lib/runtime/dart_runtime.js
index 149543b21313802e1b5d86b7c21f6dd22122e3dc..13800059810e039e0326a9ee2269690344d3eb25 100644
--- a/lib/runtime/dart_runtime.js
+++ b/lib/runtime/dart_runtime.js
@@ -302,6 +302,33 @@ var dart, _js_helper;
   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.
    */