fix how DDC finds its undefined constant

pkg/dev_compiler/test/codegen_expected/identity_test.js

 define(['dart_sdk', 'expect'], function(dart_sdk, expect) {
   'use strict';
   const core = dart_sdk.core;
   const dart = dart_sdk.dart;
   const dartx = dart_sdk.dartx;
   const minitest = expect.minitest;
   const identity_test = Object.create(null);
+  const $_get = dartx._get;
   let TToT = () => (TToT = dart.constFn(dart.gFnType(T => [T, [T]])))();
   let VoidToT = () => (VoidToT = dart.constFn(dart.gFnType(T => [T, []])))();
   let VoidToNull = () => (VoidToNull = dart.constFn(dart.fnType(core.Null, [])))();
   let VoidTodynamic = () => (VoidTodynamic = dart.constFn(dart.fnType(dart.dynamic, [])))();
   identity_test.Music = class Music extends core.Object {
     toString() {
       return {
         0: "Music.country",
         1: "Music.western"
       }[this.index];
     }
   };
   (identity_test.Music.new = function(x) {
     this.index = x;
   }).prototype = identity_test.Music.prototype;
   dart.setSignature(identity_test.Music, {
     fields: () => ({index: dart.finalFieldType(core.int)})
   });
   dart.defineEnumValues(identity_test.Music, [
     'country',
     'western'
   ]);
   identity_test.BluesBrother = class BluesBrother extends core.Object {};
   (identity_test.BluesBrother.new = function() {
   }).prototype = identity_test.BluesBrother.prototype;
+  dart.addTypeTests(identity_test.BluesBrother);
   identity_test._Jake = class _Jake extends identity_test.BluesBrother {};
   (identity_test._Jake.new = function() {
   }).prototype = identity_test._Jake.prototype;
+  dart.addTypeTests(identity_test._Jake);
   identity_test._Elwood = class _Elwood extends identity_test.BluesBrother {
     ['=='](other) {
       return identity_test._Elwood.is(other);
     }
   };
   (identity_test._Elwood.new = function() {
   }).prototype = identity_test._Elwood.prototype;
+  dart.addTypeTests(identity_test._Elwood);
   dart.setSignature(identity_test._Elwood, {
     methods: () => ({'==': dart.fnType(core.bool, [core.Object])})
   });
   identity_test._Norman = class _Norman extends identity_test.BluesBrother {};
   (identity_test._Norman.new = function() {
   }).prototype = identity_test._Norman.prototype;
+  dart.addTypeTests(identity_test._Norman);
   identity_test.hideNull = function(T) {
     return x => {
       return x;
     };
   };
   dart.fn(identity_test.hideNull, TToT());
   identity_test.getUndefined = function(T) {
     return () => {
-      return T._check(core.List.new(1)[dartx._get](0));
+      return T._check(core.List.new(1)[$_get](0));
     };
   };
   dart.fn(identity_test.getUndefined, VoidToT());
   identity_test.main = function() {
     minitest.group('Enum identity', dart.fn(() => {
       minitest.test('Identical enum/enum (nullable)', dart.fn(() => {
         let e1 = identity_test.hideNull(identity_test.Music)(identity_test.Music.country);
         let e2 = identity_test.hideNull(identity_test.Music)(identity_test.Music.western);
         let d1 = identity_test.hideNull(identity_test.Music)(identity_test.Music.country);
         let d2 = identity_test.hideNull(identity_test.Music)(identity_test.Music.western);
@@ skipping 1072 matching lines @@
   };
   dart.fn(identity_test.main, VoidTodynamic());
   dart.trackLibraries("identity_test", {
     "identity_test.dart": identity_test
   }, null);
   // Exports:
   return {
     identity_test: identity_test
   };
 });