Add stub versions of KernelInferrerEngine and friends

tests/compiler/dart2js/type_inference7_test.dart

Index: tests/compiler/dart2js/type_inference7_test.dart
diff --git a/tests/compiler/dart2js/type_inference7_test.dart b/tests/compiler/dart2js/type_inference7_test.dart
index 353085010803cb3447f36b3fed077bbb82dd8543..8875fb35c94ee0bbb0c762d8d34ea2c77a75ce6d 100644
--- a/tests/compiler/dart2js/type_inference7_test.dart
+++ b/tests/compiler/dart2js/type_inference7_test.dart
@@ -26,7 +26,7 @@ Future runTest() async {
     var typesInferrer = compiler.globalInference.typesInferrerInternal;
     var closedWorld = typesInferrer.closedWorld;
     var commonMasks = closedWorld.commonMasks;
-    dynamic foo = findElement(compiler, "foo");
+    MethodElement foo = findElement(compiler, "foo");
     // Return type is null|bool.
     var mask = typesInferrer.getReturnTypeOfMember(foo);
     Expect.isTrue(mask.isNullable);
@@ -57,7 +57,7 @@ Future runTest() async {
     var typesInferrer = compiler.globalInference.typesInferrerInternal;
     var closedWorld = typesInferrer.closedWorld;
     var commonMasks = closedWorld.commonMasks;
-    dynamic foo = findElement(compiler, "foo");
+    MethodElement foo = findElement(compiler, "foo");
     // Return type is null.
     var mask = typesInferrer.getReturnTypeOfMember(foo);
     Expect.isTrue(mask.isNullable);