Check for duplicate library names

test/codegen/expect/constructors.js

-var constructors = dart.defineLibrary(constructors, {});
-var core = dart.import(core);
-(function(exports, core) {
+dart.library('constructors', null, /* Imports */[
+  'dart/core'
+], /* Lazy imports */[
+], function(exports, core) {
   'use strict';
   class A extends core.Object {}
   class B extends core.Object {
     B() {
     }
   }
   dart.setSignature(B, {
     constructors: () => ({B: [B, []]})
   });
   class C extends core.Object {
@@ skipping 192 matching lines @@
   exports.H = H;
   exports.I = I;
   exports.J = J;
   exports.K = K;
   exports.L = L;
   exports.M = M;
   exports.N = N;
   exports.P = P;
   exports.Q$ = Q$;
   exports.Q = Q;
-})(constructors, core);
+});