Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference/infer_from_variables_in_non_cycle_imports_with_flag2.dart.strong.expect

 library test;
 import self as self;
 import "dart:core" as core;
 import "./infer_from_variables_in_non_cycle_imports_with_flag2_a.dart" as inf;
 
 class B extends core::Object {
   static field core::int y = inf::A::x;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 static method test1() → dynamic {
   inf::A::x = "hi";
   self::B::y = "hi";
 }
 static method main() → dynamic {}