fixes #131, use before define from variables to classes

test/codegen/expect/misc.js

 var misc = dart.defineLibrary(misc, {});
 var core = dart.import(core);
 (function(exports, core) {
   'use strict';
+  class _Uninitialized extends core.Object {
+    _Uninitialized() {
+    }
+  }
+  let UNINITIALIZED = dart.const(new _Uninitialized());
   // Function main: () → dynamic
   function main() {
     core.print(dart.toString(1));
     core.print(dart.toString(1.0));
     core.print(dart.toString(1.1));
   }
   // Exports:
+  exports.UNINITIALIZED = UNINITIALIZED;
   exports.main = main;
 })(misc, core);