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..5e4ba5e1854efc83b4ba779b1496d18054df4c1d 100644
--- a/test/codegen/expect/covariance.js
+++ b/test/codegen/expect/covariance.js
@@ -1,5 +1,6 @@
-var covariance;
-(function(exports) {
+var covariance = dart.defineLibrary(covariance, {});
+var core = dart.lazyImport(core);
+(function(exports, core) {
   'use strict';
   let _t = Symbol('_t');
   let Foo$ = dart.generic(function(T) {
@@ -38,4 +39,4 @@ var covariance;
   exports.Foo = Foo;
   exports.Bar = Bar;
   exports.main = main;
-})(covariance || (covariance = {}));
+})(covariance, core);