Set 'isSyntheticDefault' for default constructors.

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

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