Add stub versions of KernelInferrerEngine and friends

tests/compiler/dart2js/type_inference8_test.dart

Index: tests/compiler/dart2js/type_inference8_test.dart
diff --git a/tests/compiler/dart2js/type_inference8_test.dart b/tests/compiler/dart2js/type_inference8_test.dart
index e1876354964fd83d98771acb4b867be88b17f29c..1a77ca5b39e283af6a9421a91f12911d64f89b91 100644
--- a/tests/compiler/dart2js/type_inference8_test.dart
+++ b/tests/compiler/dart2js/type_inference8_test.dart
@@ -35,7 +35,7 @@ Future runTest1() {
   return compiler.run(uri).then((_) {
     var typesInferrer = compiler.globalInference.typesInferrerInternal;
     var commonMasks = typesInferrer.closedWorld.commonMasks;
-    var element = findElement(compiler, "foo");
+    MemberElement element = findElement(compiler, "foo");
     var mask = typesInferrer.getReturnTypeOfMember(element);
     var falseType =
         new ValueTypeMask(commonMasks.boolType, new FalseConstantValue());
@@ -78,7 +78,7 @@ Future runTest2() {
   return compiler.run(uri).then((_) {
     var typesInferrer = compiler.globalInference.typesInferrerInternal;
     var commonMasks = typesInferrer.closedWorld.commonMasks;
-    var element = findElement(compiler, "foo");
+    MemberElement element = findElement(compiler, "foo");
     var mask = typesInferrer.getReturnTypeOfMember(element);
     // Can't infer value for foo's return type, it could be either true or false
     Expect.identical(commonMasks.boolType, mask);