Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/implicit_this.dart.outline.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class C extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
   method m() → dynamic
     ;
   method testC() → dynamic
     ;
 }
 class D extends self::C {
-  constructor •() → void
+  default constructor •() → void
     ;
   method testD() → dynamic
     ;
 }
 static method main() → dynamic
   ;