Check for duplicate library names

test/codegen/expect/temps.js

-var temps = dart.defineLibrary(temps, {});
-var core = dart.import(core);
-(function(exports, core) {
+dart.library('temps.js', null, /* Imports */[
+  'dart/core.js'
+], /* Lazy imports */[
+], function(exports, core) {
   'use strict';
   let _x = Symbol('_x');
   let __x = Symbol('__x');
   let _function = Symbol('_function');
   class FormalCollision extends core.Object {
     FormalCollision(x, _x$, func) {
       this[_x] = x;
       this[__x] = _x$;
       this[_function] = func;
     }
@@ skipping 23 matching lines @@
   function main() {
     core.print(new FormalCollision(1, 2, dart.fn(x => x)));
     core.print(new OptionalArg()[_opt]);
     core.print(new OptionalArg.named()[_opt]);
   }
   dart.fn(main);
   // Exports:
   exports.FormalCollision = FormalCollision;
   exports.OptionalArg = OptionalArg;
   exports.main = main;
-})(temps, core);
+});