Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/rasta/issue_000012.dart.direct.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class A extends core::Object {
   field dynamic field = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class B extends self::A {
-  constructor •() → void
+  default constructor •() → void
     : super self::A::•()
     ;
   method m() → dynamic {
     super.{self::A::field} = 42;
   }
 }
 static method main() → dynamic {
   new self::B::•().m();
 }