Set 'isSyntheticDefault' for default constructors.

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

 library test;
 import self as self;
 import "dart:core" as core;
 
 class C extends core::Object {
   static final field (core::bool) → (core::int) → core::Map<core::int, core::bool> f = (core::bool b) → (core::int) → core::Map<core::int, core::bool> => (core::int i) → core::Map<core::int, core::bool> => <core::int, core::bool>{i: b};
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 static method main() → dynamic {}