Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference/assign_local.dart.strong.expect

Index: pkg/front_end/testcases/inference/assign_local.dart.strong.expect
diff --git a/pkg/front_end/testcases/inference/assign_local.dart.strong.expect b/pkg/front_end/testcases/inference/assign_local.dart.strong.expect
index 6014421072be895c1fd7262ff11efe04f7133a4d..e3529d7e2021c56cce60a62977230aa3f61bc00d 100644
--- a/pkg/front_end/testcases/inference/assign_local.dart.strong.expect
+++ b/pkg/front_end/testcases/inference/assign_local.dart.strong.expect
@@ -3,12 +3,12 @@ import self as self;
 import "dart:core" as core;
 
 class A<T extends core::Object> extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class B<T extends core::Object> extends self::A<self::B::T> {
-  constructor •() → void
+  default constructor •() → void
     : super self::A::•()
     ;
 }