Set 'isSyntheticDefault' for default constructors.

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

 library test;
 import self as self;
 import "dart:core" as core;
 
 class C extends core::Object {
   field core::int x = 0;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class D extends self::C {
-  constructor •() → void
+  default constructor •() → void
     : super self::C::•()
     ;
   method g() → void {
     core::int y = super.{self::C::x};
   }
 }
 static method main() → dynamic {}