Check for duplicate library names

lib/runtime/dart/_js_primitives.js

-var _js_primitives = dart.defineLibrary(_js_primitives, {});
-var core = dart.import(core);
+dart.defineLibrary('_js_primitives');
+dart.import('core');
 (function(exports, core) {
   'use strict';
   function printString(string) {
     if (typeof dartPrint == "function") {
       dartPrint(string);
       return;
     }
     if (typeof console == "object" && typeof console.log != "undefined") {
       console.log(string);
       return;
     }
     if (typeof window == "object") {
       return;
     }
     if (typeof print == "function") {
       print(string);
       return;
     }
     throw "Unable to print message: " + String(string);
   }
   dart.fn(printString, dart.void, [core.String]);
   // Exports:
   exports.printString = printString;
 })(_js_primitives, core);