Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference_new/infer_instance_field_ref_circular.dart.direct.expect

 library test;
 import self as self;
 import "dart:core" as core;
 
 class A extends core::Object {
   field dynamic b = () → dynamic => self::x;
   field dynamic c = () → dynamic => self::x;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 static field dynamic a = new self::A::•();
 static field dynamic x = () → dynamic => self::a.b;
 static field dynamic y = () → dynamic => self::a.c;
 static method main() → dynamic {}