fixes #157, renaming local library identifiers if needed.

test/codegen/expect/constructors.js

Index: test/codegen/expect/constructors.js
diff --git a/test/codegen/expect/constructors.js b/test/codegen/expect/constructors.js
index 4fb06b7c78bdcf4b32eb76d6eb90753fddf2edc8..4033ffff354e13cee91eebad5d0b7dc2f949bef5 100644
--- a/test/codegen/expect/constructors.js
+++ b/test/codegen/expect/constructors.js
@@ -1,5 +1,6 @@
-var constructors;
-(function(exports) {
+var constructors = dart.defineLibrary(constructors, {});
+var core = dart.lazyImport(core);
+(function(exports, core) {
   'use strict';
   class A extends core.Object {}
   class B extends core.Object {
@@ -116,4 +117,4 @@ var constructors;
   exports.M = M;
   exports.N = N;
   exports.P = P;
-})(constructors || (constructors = {}));
+})(constructors, core);