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

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 97e39946de2dc4c2e610dec147d1cc828c4974ae..114b144db9cd0e80881847c32c0c60e1330e124a 100644
--- a/lib/runtime/dart/_runtime.js
+++ b/lib/runtime/dart/_runtime.js
@@ -726,7 +726,8 @@ dart_library.library('dart/_runtime', null, /* Imports */[
   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();
     }