Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/rasta/issue_000026.dart.direct.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class C extends core::Object {
   field dynamic a = null;
   field dynamic b = 0;
   field dynamic c = 1.+(2);
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class D extends core::Object {
   field dynamic a = null;
   field dynamic b = 1;
   field dynamic c = 2.-(3);
   constructor •() → void
     : super core::Object::•()
     ;
 }
 static method main() → dynamic {
   new self::C::•();
   new self::D::•();
 }