Check for duplicate library names

test/codegen/expect/functions.js

-var functions = dart.defineLibrary(functions, {});
-var core = dart.import(core);
-(function(exports, core) {
+dart.library('functions', null, /* Imports */[
+  'dart/core'
+], /* Lazy imports */[
+], function(exports, core) {
   'use strict';
   function bootstrap() {
     return dart.list([new Foo()], Foo);
   }
   dart.fn(bootstrap, () => dart.functionType(core.List$(Foo), []));
   let A2B$ = dart.generic(function(A, B) {
     let A2B = dart.typedef('A2B', () => dart.functionType(B, [A]));
     return A2B;
   });
   let A2B = A2B$();
   function id(f) {
     return f;
   }
   dart.fn(id, () => dart.functionType(A2B$(Foo, Foo), [A2B$(Foo, Foo)]));
   class Foo extends core.Object {}
   function main() {
     core.print(bootstrap()[dartx.get](0));
   }
   dart.fn(main, dart.void, []);
   // Exports:
   exports.bootstrap = bootstrap;
   exports.A2B$ = A2B$;
   exports.A2B = A2B;
   exports.id = id;
   exports.Foo = Foo;
   exports.main = main;
-})(functions, core);
+});