fix how DDC finds its undefined constant

pkg/dev_compiler/test/codegen_expected/es6_modules.js

Index: pkg/dev_compiler/test/codegen_expected/es6_modules.js
diff --git a/pkg/dev_compiler/test/codegen_expected/es6_modules.js b/pkg/dev_compiler/test/codegen_expected/es6_modules.js
index 25fc96b9791d2c145b3c5000dc3c23a512b9848a..46bc5340500082eef2c688dd2e6101723a5c9fd6 100644
--- a/pkg/dev_compiler/test/codegen_expected/es6_modules.js
+++ b/pkg/dev_compiler/test/codegen_expected/es6_modules.js
@@ -8,25 +8,33 @@ es6_modules.Callback = dart.typedef('Callback', () => dart.fnTypeFuzzy(dart.void
 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());