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 47ef79427ce9949670b666f8ff9e6b7ce6307c95..8a986bce275276f0102decb8e2f3f7ba3c1925a4 100644
--- a/lib/runtime/dart/_runtime.js
+++ b/lib/runtime/dart/_runtime.js
@@ -732,7 +732,8 @@ 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();
     }