Check for duplicate library names

test/codegen/expect/domtest.js

-var domtest = dart.defineLibrary(domtest, {});
-var dom = dart.import(dom);
-var core = dart.import(core);
-(function(exports, dom, core) {
+dart.library('domtest', null, /* Imports */[
+  'sunflower/dom',
+  'dart/core'
+], /* Lazy imports */[
+], function(exports, dom, core) {
   'use strict';
   function testNativeIndexers() {
     let nodes = dom.document.querySelector('body').childNodes;
     for (let i = 0; dart.notNull(i) < dart.notNull(nodes.length); i = dart.notNull(i) + 1) {
       let old = nodes[i];
       nodes[i] = dom.document.createElement('div');
       core.print(dart.equals(nodes[i], old));
     }
   }
   dart.fn(testNativeIndexers);
   // Exports:
   exports.testNativeIndexers = testNativeIndexers;
-})(domtest, dom, core);
+});