Set 'isSyntheticDefault' for default constructors.

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

 library test;
 import self as self;
 import "dart:core" as core;
 
 class C extends core::Object {
   final field core::int x = self::y;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 static get y() → core::int
   return null;
 static method main() → dynamic {}