Set 'isSyntheticDefault' for default constructors.

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

 library test;
 import self as self;
 import "dart:core" as core;
 
 class A extends core::Object {
   field core::int x = 2;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 static method main() → dynamic {
   self::A a = new self::A::•();
   self::A b = a;
   core::print(a.{self::A::x});
   core::print(a.{self::A::x}.{core::num::+}(2));
 }