Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/accessors.dart.direct.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class C extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   set onlySetter(dynamic value) → void {
     core::print("C.onlySetter called with ${value}.");
   }
   method testC() → dynamic {
     try {
       core::print(this.onlySetter);
       throw "No error thrown";
     }
     on core::NoSuchMethodError catch(final core::NoSuchMethodError e) {
       core::print("Expected error: ${e}");
     }
     this.onlySetter = "hest";
   }
   method testD() → dynamic {
     core::print(this.onlySetter);
     this.onlySetter = "hest";
   }
 }
 class D extends self::C {
-  constructor •() → void
+  default constructor •() → void
     : super self::C::•()
     ;
   get onlySetter() → core::String
     return "D.onlySetter called.";
   set onlySetter(dynamic value) → void {
     core::print("D.onlySetter called with ${value}.");
   }
 }
 static set onlySetter(dynamic value) → void {
   core::print("onlySetter called with ${value}.");
 }
 static method main() → dynamic {
   try {
     core::print(throw new core::NoSuchMethodError::_withType(null, #onlySetter, 33, <dynamic>[].toList(growable: false), <dynamic, dynamic>{}, null));
     throw "No error thrown";
   }
   on core::NoSuchMethodError catch(final core::NoSuchMethodError e) {
     core::print("Expected error: ${e}");
   }
   self::onlySetter = "fisk";
   new self::C::•().testC();
   new self::D::•().testD();
 }