Check for duplicate library names

test/codegen/expect/sunflower/dom.js

-var dom = dart.defineLibrary(dom, window);

[vsm, 2015/06/04 22:36:53]

Note, in the old pattern, this is silently broken if dom was already defined by
a lazyImport - we ignore window.

In this CL, it's still broken, but we throw a runtime error.

-var core = dart.import(core);
+dart.defineLibrary('dom', window);
+dart.import('core');
 (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 29 matching lines @@
   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);