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

test/codegen/expect/closure.js

 dart_library.library('closure', null, /* Imports */[
   'dart/_runtime',
   'dart/core',
   'dart/js'
 ], /* Lazy imports */[
 ], function(exports, dart, core, js) {
   'use strict';
   let dartx = dart.dartx;
   /** @typedef {function({i: (?number|undefined)}=)} */
   const Callback = dart.typedef('Callback', () => dart.functionType(dart.void, [], {i: core.int}));
@@ skipping 99 matching lines @@
     /** @final {string} */
     Foo.some_static_constant = "abc";
     /** @final {string} */
     Foo.some_static_final = "abc";
     /** @type {string} */
     Foo.some_static_var = "abc";
     return Foo;
   });
   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$super {
     /** @param {?number} i */
     Baz(i) {
       super.Foo(i, 123);
     }
   }
   dart.setSignature(Baz, {
     constructors: () => ({Baz: [Baz, [core.int]]})
   });
   /** @param {?} args */
   function main(args) {
   }
   dart.fn(main, dart.void, [dart.dynamic]);
   /** @final {string} */
   const some_top_level_constant = "abc";
   /** @final {string} */
   exports.some_top_level_final = "abc";
   /** @type {string} */
   exports.some_top_level_var = "abc";
   // Exports:
   exports.Callback = Callback;
   exports.Foo$ = Foo$;
   exports.Foo = Foo;
   exports.Bar = Bar;
   exports.Baz = Baz;
   exports.main = main;
   exports.some_top_level_constant = some_top_level_constant;
 });