fix how DDC finds its undefined constant

pkg/dev_compiler/test/codegen_expected/BenchmarkBase.js

 define(['dart_sdk'], function(dart_sdk) {
   'use strict';
   const core = dart_sdk.core;
   const dart = dart_sdk.dart;
   const dartx = dart_sdk.dartx;
   const BenchmarkBase$ = Object.create(null);
+  const $length = dartx.length;
+  const $_get = dartx._get;
   let VoidToNull = () => (VoidToNull = dart.constFn(dart.fnType(core.Null, [])))();
   BenchmarkBase$.Expect = class Expect extends core.Object {
     static equals(expected, actual) {
       if (!dart.equals(expected, actual)) {
         dart.throw(dart.str`Values not equal: ${expected} vs ${actual}`);
       }
     }
     static listEquals(expected, actual) {
-      if (expected[dartx.length] != actual[dartx.length]) {
-        dart.throw(dart.str`Lists have different lengths: ${expected[dartx.length]} vs ${actual[dartx.length]}`);
+      if (expected[$length] != actual[$length]) {
+        dart.throw(dart.str`Lists have different lengths: ${expected[$length]} vs ${actual[$length]}`);
       }
-      for (let i = 0; i < dart.notNull(actual[dartx.length]); i++) {
-        BenchmarkBase$.Expect.equals(expected[dartx._get](i), actual[dartx._get](i));
+      for (let i = 0; i < dart.notNull(actual[$length]); i++) {
+        BenchmarkBase$.Expect.equals(expected[$_get](i), actual[$_get](i));
       }
     }
     fail(message) {
       dart.throw(message);
     }
   };
   (BenchmarkBase$.Expect.new = function() {
   }).prototype = BenchmarkBase$.Expect.prototype;
+  dart.addTypeTests(BenchmarkBase$.Expect);
   dart.setSignature(BenchmarkBase$.Expect, {
     methods: () => ({fail: dart.fnType(dart.dynamic, [dart.dynamic])}),
     statics: () => ({
       equals: dart.fnType(dart.void, [dart.dynamic, dart.dynamic]),
       listEquals: dart.fnType(dart.void, [core.List, core.List])
     }),
     names: ['equals', 'listEquals']
   });
   BenchmarkBase$.BenchmarkBase = class BenchmarkBase extends core.Object {
     get name() {
@@ skipping 38 matching lines @@
       return result;
     }
     report() {
       let score = this.measure();
       core.print(dart.str`${this.name}(RunTime): ${score} us.`);
     }
   };
   (BenchmarkBase$.BenchmarkBase.new = function(name) {
     this[name$] = name;
   }).prototype = BenchmarkBase$.BenchmarkBase.prototype;
+  dart.addTypeTests(BenchmarkBase$.BenchmarkBase);
   const name$ = Symbol("BenchmarkBase.name");
   dart.setSignature(BenchmarkBase$.BenchmarkBase, {
     fields: () => ({name: dart.finalFieldType(core.String)}),
     methods: () => ({
       run: dart.fnType(dart.void, []),
       warmup: dart.fnType(dart.void, []),
       exercise: dart.fnType(dart.void, []),
       setup: dart.fnType(dart.void, []),
       teardown: dart.fnType(dart.void, []),
       measure: dart.fnType(core.double, []),
       report: dart.fnType(dart.void, [])
     }),
     statics: () => ({measureFor: dart.fnType(core.double, [core.Function, core.int])}),
     names: ['measureFor']
   });
   dart.trackLibraries("BenchmarkBase", {
     "BenchmarkBase.dart": BenchmarkBase$
   }, null);
   // Exports:
   return {
     BenchmarkBase: BenchmarkBase$
   };
 });