Set 'isSyntheticDefault' for default constructors.

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

 library test;
 import self as self;
 import "dart:core" as core;
 
 class C extends self::D {
-  constructor •() → void
+  default constructor •() → void
     : super self::D::•()
     ;
   set foo(core::int x) → void {}
 }
 class D extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   set foo(core::int x) → void {}
 }
 static method main() → dynamic {}