Check for duplicate library names

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 e5eb3d3a5840a2349cdcd7647a2255f9fc5d7bb7..9e94201a2f62ced4c95275b7568ac4aac9a09639 100644
--- a/test/codegen/expect/covariance.js
+++ b/test/codegen/expect/covariance.js
@@ -1,5 +1,5 @@
-var covariance = dart.defineLibrary(covariance, {});
-var core = dart.import(core);
+dart.defineLibrary('covariance');
+dart.import('core');
 (function(exports, core) {
   'use strict';
   let _t = Symbol('_t');