Add stub versions of KernelInferrerEngine and friends

tests/compiler/dart2js/closure_tracer_test.dart

Index: tests/compiler/dart2js/closure_tracer_test.dart
diff --git a/tests/compiler/dart2js/closure_tracer_test.dart b/tests/compiler/dart2js/closure_tracer_test.dart
index b39cae347327d4d713647b289ecf0ca583b2c7af..c9f519b36661697692ba755ae6682d995bb5c7db 100644
--- a/tests/compiler/dart2js/closure_tracer_test.dart
+++ b/tests/compiler/dart2js/closure_tracer_test.dart
@@ -159,7 +159,7 @@ void main() {
         var commonMasks = closedWorld.commonMasks;
 
         checkType(String name, type) {
-          var element = findElement(compiler, name);
+          MemberElement element = findElement(compiler, name);
           var mask = typesInferrer.getReturnTypeOfMember(element);
           Expect.equals(type.nullable(), simplify(mask, closedWorld), name);
         }