fix how DDC finds its undefined constant

pkg/dev_compiler/test/codegen_expected/es6_modules.js

 export const es6_modules = Object.create(null);
 import { core, dart, dartx } from 'dart_sdk';
 let B = () => (B = dart.constFn(es6_modules.B$()))();
 let _B = () => (_B = dart.constFn(es6_modules._B$()))();
 let VoidTodynamic = () => (VoidTodynamic = dart.constFn(dart.fnType(dart.dynamic, [])))();
 let VoidToString = () => (VoidToString = dart.constFn(dart.fnType(core.String, [])))();
 es6_modules.Callback = dart.typedef('Callback', () => dart.fnTypeFuzzy(dart.void, [], {i: core.int}));
 es6_modules.A = class A extends core.Object {};
 (es6_modules.A.new = function() {
 }).prototype = es6_modules.A.prototype;
+dart.addTypeTests(es6_modules.A);
 es6_modules._A = class _A extends core.Object {};
 (es6_modules._A.new = function() {
 }).prototype = es6_modules._A.prototype;
+dart.addTypeTests(es6_modules._A);
+const _is_B_default = Symbol('_is_B_default');
 es6_modules.B$ = dart.generic(T => {
   class B extends core.Object {}
   (B.new = function() {
   }).prototype = B.prototype;
   dart.addTypeTests(B);
+  B.prototype[_is_B_default] = true;
   return B;
 });
 es6_modules.B = B();
+dart.addTypeTests(es6_modules.B, _is_B_default);
+const _is__B_default = Symbol('_is__B_default');
 es6_modules._B$ = dart.generic(T => {
   class _B extends core.Object {}
   (_B.new = function() {
   }).prototype = _B.prototype;
   dart.addTypeTests(_B);
+  _B.prototype[_is__B_default] = true;
   return _B;
 });
 es6_modules._B = _B();
+dart.addTypeTests(es6_modules._B, _is__B_default);
 es6_modules.f = function() {
 };
 dart.fn(es6_modules.f, VoidTodynamic());
 es6_modules._f = function() {
 };
 dart.fn(es6_modules._f, VoidTodynamic());
 dart.defineLazy(es6_modules, {
   get constant() {
     return "abc";
   },
@@ skipping 14 matching lines @@
     return dart.fn(() => {
       core.print('lazyMutable');
       return "abc";
     }, VoidToString())();
   },
   set lazyMutable(_) {}
 });
 dart.trackLibraries("es6_modules", {
   "es6_modules.dart": es6_modules
 }, null);