Add stub versions of KernelInferrerEngine and friends

tests/compiler/dart2js/type_inference_switch_test.dart

Index: tests/compiler/dart2js/type_inference_switch_test.dart
diff --git a/tests/compiler/dart2js/type_inference_switch_test.dart b/tests/compiler/dart2js/type_inference_switch_test.dart
index 56e468ed7e076ac8bfe4a555f22310e6eb9f9843..8b0b8e46e064ebf73c6d00f722374c9036745ea6 100644
--- a/tests/compiler/dart2js/type_inference_switch_test.dart
+++ b/tests/compiler/dart2js/type_inference_switch_test.dart
@@ -139,7 +139,7 @@ Future runTest(String test, checker) {
     var commonMasks = closedWorld.commonMasks;
 
     checkTypeOf(String name, TypeMask type) {
-      var element = findElement(compiler, name);
+      MemberElement element = findElement(compiler, name);
       var mask = typesInferrer.getReturnTypeOfMember(element);
       Expect.equals(type, simplify(mask, closedWorld));
     }