Set 'isSyntheticDefault' for default constructors.

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

 library;
 import self as self;
 import "dart:core" as core;
 
 class Mixin extends core::Object {
   field dynamic field = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 abstract class Object&Mixin extends core::Object implements self::Mixin {
   field dynamic field = null;
   constructor •() → void
     : super core::Object::•()
     ;
 }
 abstract class Object&Mixin&Mixin extends self::Object&Mixin implements self::Mixin {
   field dynamic field = null;
   constructor •() → void
     : super self::Object&Mixin::•()
     ;
 }
 class A extends self::Object&Mixin&Mixin {
-  constructor •() → void
+  default constructor •() → void
     : super self::Object&Mixin&Mixin::•()
     ;
 }