Check for duplicate library names

test/codegen/expect/methods.js

-var methods = dart.defineLibrary(methods, {});
-var core = dart.import(core);
-(function(exports, core) {
+dart.library('methods', null, /* Imports */[
+  'dart/core'
+], /* Lazy imports */[
+], function(exports, core) {
   'use strict';
   let _c = Symbol('_c');
   class A extends core.Object {
     A() {
       this[_c] = 3;
     }
     x() {
       return 42;
     }
     y(a) {
@@ skipping 58 matching lines @@
     let g = dart.bind(a, 'x');
     let aa = new A();
     let h = dart.dload(aa, 'x');
   }
   dart.fn(test);
   // Exports:
   exports.A = A;
   exports.Bar = Bar;
   exports.Foo = Foo;
   exports.test = test;
-})(methods, core);
+});