Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference/greatest_closure_multiple_params.dart.direct.expect

 library test;
 import self as self;
 import "dart:core" as core;
 
 abstract class C<E extends core::Object> extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   method sort([(self::C::E, self::C::E) → core::int compare = null]) → void {
     self::C::sort2<dynamic>(this, let final dynamic #t1 = compare in #t1.==(null) ? self::C::_compareAny : #t1);
   }
   static method _compareAny(dynamic a, dynamic b) → core::int {
     throw "unimplemented";
   }
   static method sort2<E extends core::Object>(self::C<self::C::sort2::E> a, (self::C::sort2::E, self::C::sort2::E) → core::int compare) → void {
     throw "unimplemented";
   }
 }
 static method main() → dynamic {}