Add stub versions of KernelInferrerEngine and friends

tests/compiler/dart2js/trust_type_annotations_test.dart

Index: tests/compiler/dart2js/trust_type_annotations_test.dart
diff --git a/tests/compiler/dart2js/trust_type_annotations_test.dart b/tests/compiler/dart2js/trust_type_annotations_test.dart
index 55514ab32516b98d5a8c85ed705cdd6e2e30f0be..a804f0e6b011fde671f8a0a3f728f9c0ec269c97 100644
--- a/tests/compiler/dart2js/trust_type_annotations_test.dart
+++ b/tests/compiler/dart2js/trust_type_annotations_test.dart
@@ -56,13 +56,13 @@ void main() {
         ClassElement classA = findElement(compiler, "A");
 
         checkReturn(String name, TypeMask type) {
-          var element = classA.lookupMember(name);
+          MemberElement element = classA.lookupMember(name);
           var mask = typesInferrer.getReturnTypeOfMember(element);
           Expect.isTrue(type.containsMask(mask, closedWorld));
         }
 
         checkType(String name, type) {
-          var element = classA.lookupMember(name);
+          MemberElement element = classA.lookupMember(name);
           Expect.isTrue(type.containsMask(
               typesInferrer.getTypeOfMember(element), closedWorld));
         }