Create local alias for super class in --closure mode (issue #312)

lib/runtime/dart/_runtime.js

 dart_library.library('dart/_runtime', null, /* Imports */[
 ], /* Lazy imports */[
   'dart/core',
   'dart/_interceptors',
   'dart/_js_helper',
   'dart/async',
   'dart/collection'
 ], function(exports, core, _interceptors, _js_helper, async, collection) {
   'use strict';
   function mixin(base, ...mixins) {
@@ skipping 714 matching lines @@
       if (compute == null) return cache;
       cache = compute();
       compute = null;
       return cache;
     }
     tagComputed(value, getter);
   }
   const _mixins = Symbol("mixins");
   const implements_ = Symbol("implements");
   const metadata = Symbol("metadata");
-  const TypeRep = class TypeRep extends LazyTagged(() => core.Type) {
+  const _TypeRepBase = LazyTagged(() => core.Type);
+  const TypeRep = class TypeRep extends _TypeRepBase {
     get name() {
       return this.toString();
     }
   };
   const Dynamic = class Dynamic extends TypeRep {
     toString() {
       return "dynamic";
     }
   };
   const dynamicR = new Dynamic();
@@ skipping 554 matching lines @@
   exports.copyProperties = copyProperties;
   exports.export = export_;
   exports.defineLazyClass = defineLazyClass;
   exports.defineLazyProperties = defineLazyProperties;
   exports.defineLazyClassGeneric = defineLazyClassGeneric;
   exports.as = as_;
   exports.is = is_;
   exports.global = global_;
   exports.JsSymbol = JsSymbol;
 });