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 dedd188764341ba0b9d768a3127e44b2c8021013..a851e72da622d3075bf25eef8e6670a9d38c8ec4 100644
--- a/test/codegen/expect/closure.js
+++ b/test/codegen/expect/closure.js
@@ -117,7 +117,8 @@ dart_library.library('closure', null, /* Imports */[
   /** @type {string} */
   Foo.some_static_var = "abc";
   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);