fixes #131, use before define from variables to classes

test/codegen/expect/constructors.js

 var constructors = dart.defineLibrary(constructors, {});
 var core = dart.import(core);
 (function(exports, core) {
   'use strict';
   class A extends core.Object {}
   class B extends core.Object {
     B() {
     }
   }
   class C extends core.Object {
@@ skipping 91 matching lines @@
   dart.defineNamedConstructor(P, 'bar');
   let Q$ = dart.generic(function(T) {
     class Q extends core.Object {
       Q(y) {
         this.x = dart.as(y, T);
       }
       static foo() {
         return new (Q$())("hello");
       }
       bar() {
-        let q = Q.foo();
+        let q = Q$().foo();
         return dart.as(q.x, core.String);
       }
       bar2() {
         let q = new (Q$())("world");
         return dart.as(q.x, core.String);
       }
       static baz() {
         let q = new (Q$(core.int))(42);
         return dart.notNull(q.bar()) + dart.notNull(q.bar2());
       }
@@ skipping 14 matching lines @@
   exports.I = I;
   exports.J = J;
   exports.K = K;
   exports.L = L;
   exports.M = M;
   exports.N = N;
   exports.P = P;
   exports.Q$ = Q$;
   exports.Q = Q;
 })(constructors, core);