Set 'isSyntheticDefault' for default constructors.

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

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