Check for duplicate library names

test/codegen/expect/sunflower/painter.js

-var painter = dart.defineLibrary(painter, {});
-var math = dart.import(math);
-var dom = dart.import(dom);
-var core = dart.import(core);
-var circle = dart.import(circle);
+dart.defineLibrary('painter');
+dart.import('math');
+dart.import('dom');
+dart.import('core');
+dart.import('circle');
 (function(exports, math, dom, core, circle) {
   'use strict';
   let ORANGE = "orange";
   let RED = "red";
   let BLUE = "blue";
   let TAU = dart.notNull(math.PI) * 2;
   function querySelector(selector) {
     return dom.document.querySelector(selector);
   }
   dart.fn(querySelector, dom.Element, [core.String]);
@@ skipping 25 matching lines @@
     methods: () => ({draw: [dart.void, [dom.CanvasRenderingContext2D]]})
   });
   // Exports:
   exports.ORANGE = ORANGE;
   exports.RED = RED;
   exports.BLUE = BLUE;
   exports.TAU = TAU;
   exports.querySelector = querySelector;
   exports.CirclePainter = CirclePainter;
 })(painter, math, dom, core, circle);