Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference/infer_field_from_later_inferred_field.dart.direct.expect

 library test;
 import self as self;
 import "dart:core" as core;
 
 class A extends core::Object implements self::B {
   field dynamic x = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class B extends core::Object {
   field dynamic x = 0;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 static method main() → dynamic {}