Check for duplicate library names

test/codegen/expect/sunflower/sunflower.js

-var sunflower = dart.defineLibrary(sunflower, {});
-var dom = dart.import(dom);
-var core = dart.import(core);
-var math = dart.import(math);
-var painter = dart.import(painter);
-var circle = dart.import(circle);
-(function(exports, dom, core, math, painter, circle) {
+dart.library('sunflower', null, /* Imports */[
+  'dom',
+  'dart/core',
+  'dart/math',
+  'painter',
+  'circle'
+], /* Lazy imports */[
+], function(exports, dom, core, math, painter, circle) {
   'use strict';
   let SEED_RADIUS = 2;
   let SCALE_FACTOR = 4;
   let MAX_D = 300;
   let centerX = dart.notNull(MAX_D) / 2;
   let centerY = centerX;
   function querySelector(selector) {
     return dom.document.querySelector(selector);
   }
   dart.fn(querySelector, dom.Element, [core.String]);
@@ skipping 48 matching lines @@
   // Exports:
   exports.SEED_RADIUS = SEED_RADIUS;
   exports.SCALE_FACTOR = SCALE_FACTOR;
   exports.MAX_D = MAX_D;
   exports.centerX = centerX;
   exports.centerY = centerY;
   exports.querySelector = querySelector;
   exports.main = main;
   exports.draw = draw;
   exports.SunflowerSeed = SunflowerSeed;
-})(sunflower, dom, core, math, painter, circle);
+});