Check for duplicate library names

test/codegen/expect/sunflower/dom.js

-var dom = dart.defineLibrary(dom, window);
-var core = dart.import(core);
-(function(exports, core) {
+dart.library('dom.js', window, /* Imports */[
+  'dart/core.js'
+], /* Lazy imports */[
+], function(exports, core) {
   'use strict';
   class JsName extends core.Object {
     JsName(opts) {
       let name = opts && 'name' in opts ? opts.name : null;
       this.name = name;
     }
   }
   dart.setSignature(JsName, {
     constructors: () => ({JsName: [JsName, [], {name: core.String}]})
   });
@@ skipping 27 matching lines @@
   exports.JsName = JsName;
   exports.Overload = Overload;
   exports.overload = overload;
   exports.EventListener = EventListener;
   exports.Event = Event;
   exports.InputElement = InputElement;
   exports.CanvasElement = CanvasElement;
   exports.RenderingContext = RenderingContext;
   exports.CanvasDrawingStyles = CanvasDrawingStyles;
   exports.CanvasPathMethods = CanvasPathMethods;
-})(dom, core);
+});