fixes #157, renaming local library identifiers if needed.

test/codegen/expect/methods.js

Index: test/codegen/expect/methods.js
diff --git a/test/codegen/expect/methods.js b/test/codegen/expect/methods.js
index 87611b28f46ae68001dd5b444540fe235628c2d7..222970e71c12783aef6c4a375d8bc0f076e2d913 100644
--- a/test/codegen/expect/methods.js
+++ b/test/codegen/expect/methods.js
@@ -1,5 +1,6 @@
-var methods;
-(function(exports) {
+var methods = dart.defineLibrary(methods, {});
+var core = dart.import(core);
+(function(exports, core) {
   'use strict';
   let _c = Symbol('_c');
   class A extends core.Object {
@@ -61,4 +62,4 @@ var methods;
   exports.Bar = Bar;
   exports.Foo = Foo;
   exports.test = test;
-})(methods || (methods = {}));
+})(methods, core);