fixes #157, renaming local library identifiers if needed.

test/codegen/expect/covariance.js

-var covariance;
-(function(exports) {
+var covariance = dart.defineLibrary(covariance, {});
+var core = dart.import(core);
+(function(exports, core) {
   'use strict';
   let _t = Symbol('_t');
   let Foo$ = dart.generic(function(T) {
     class Foo extends core.Object {
       Foo() {
         this[_t] = null;
       }
       add(t) {
         dart.as(t, T);
         this[_t] = t;
@@ skipping 18 matching lines @@
   // Function main: () → dynamic
   function main() {
     let foo = new Bar();
     foo.add('hi');
   }
   // Exports:
   exports.Foo$ = Foo$;
   exports.Foo = Foo;
   exports.Bar = Bar;
   exports.main = main;
-})(covariance || (covariance = {}));
+})(covariance, core);