Set 'isSyntheticDefault' for default constructors.

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

 library test;
 import self as self;
 import "dart:core" as core;
 import "./infer_statics_with_method_invocations_a.dart" as inf;
 
 class T extends core::Object {
   static final field self::T foo = self::T::m1(self::T::m2(inf::m3("", "")));
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   static method m1(core::String m) → self::T {
     return null;
   }
   static method m2(dynamic e) → core::String {
     return "";
   }
 }
 static method main() → dynamic {}