Add stub versions of KernelInferrerEngine and friends

tests/compiler/dart2js/map_tracer_keys_test.dart

Index: tests/compiler/dart2js/map_tracer_keys_test.dart
diff --git a/tests/compiler/dart2js/map_tracer_keys_test.dart b/tests/compiler/dart2js/map_tracer_keys_test.dart
index 621466af2531e16b6bd37c74182d5cebed8d764b..d5ee14f6d8824b743101680392716b471a5651e0 100644
--- a/tests/compiler/dart2js/map_tracer_keys_test.dart
+++ b/tests/compiler/dart2js/map_tracer_keys_test.dart
@@ -59,10 +59,10 @@ void doTest(
   asyncTest(() => compiler.run(uri).then((_) {
         var typesInferrer = compiler.globalInference.typesInferrerInternal;
         var commonMasks = typesInferrer.closedWorld.commonMasks;
-        var aDoubleType =
-            typesInferrer.getTypeOfMember(findElement(compiler, 'aDouble'));
-        var aListType =
-            typesInferrer.getTypeOfMember(findElement(compiler, 'aList'));
+        MemberElement aDouble = findElement(compiler, 'aDouble');
+        var aDoubleType = typesInferrer.getTypeOfMember(aDouble);
+        MemberElement aList = findElement(compiler, 'aList');
+        var aListType = typesInferrer.getTypeOfMember(aList);
 
         Expect.equals(aDoubleType, commonMasks.doubleType);
         Expect.isTrue(aListType is ContainerTypeMask);