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

test/codegen/expect/closure.js

Index: test/codegen/expect/closure.js
diff --git a/test/codegen/expect/closure.js b/test/codegen/expect/closure.js
index b33e351fec2f42119f7e7e2b68a55de6ff99028f..f8084ab244953329ef1201a7876ba50af255d0b2 100644
--- a/test/codegen/expect/closure.js
+++ b/test/codegen/expect/closure.js
@@ -117,7 +117,8 @@ dart_library.library('closure', null, /* Imports */[
   });
   let Foo = Foo$();
   class Bar extends core.Object {}
-  class Baz extends dart.mixin(Foo$(core.int), Bar) {
+  const Baz$super = dart.mixin(Foo$(core.int), Bar);
+  class Baz extends Baz$super {
     /** @param {?number} i */
     Baz(i) {
       super.Foo(i, 123);