Set 'isSyntheticDefault' for default constructors.

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

 library test;
 import self as self;
 import "dart:core" as core;
 
 class A extends core::Object {
   static field dynamic x = null;
   static field core::int y = 3;
   field dynamic x2 = null;
   field core::int y2 = 3;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 static field dynamic x = null;
 static field core::int y = 3;
 static method main() → dynamic {
   self::x;
   self::y;
 }