Check for duplicate library names

test/codegen/expect/sunflower/circle.js

-var circle = dart.defineLibrary(circle, {});
-var core = dart.import(core);
+dart.defineLibrary('circle');
+dart.import('core');
 (function(exports, core) {
   'use strict';
   class Circle extends core.Object {
     Circle(x, y, radius) {
       this.x = x;
       this.y = y;
       this.radius = radius;
     }
   }
   dart.setSignature(Circle, {
     constructors: () => ({Circle: [Circle, [core.num, core.num, core.num]]})
   });
   // Exports:
   exports.Circle = Circle;
 })(circle, core);