fixes #157, renaming local library identifiers if needed.

test/codegen/expect/covariance.js

Index: test/codegen/expect/covariance.js
diff --git a/test/codegen/expect/covariance.js b/test/codegen/expect/covariance.js
index e3d53be0aeaa3e435ba06864959541bd30a32626..f341a8a97502c5a18e2850de3b2fe302b091c692 100644
--- a/test/codegen/expect/covariance.js
+++ b/test/codegen/expect/covariance.js
@@ -1,5 +1,5 @@
-var covariance;
-(function(exports) {
+var covariance, core;
+(function(exports, core) {
   'use strict';
   let _t = Symbol('_t');
   let Foo$ = dart.generic(function(T) {
@@ -38,4 +38,4 @@ var covariance;
   exports.Foo = Foo;
   exports.Bar = Bar;
   exports.main = main;
-})(covariance || (covariance = {}));
+})(covariance || (covariance = {}), core);