Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference/inferred_type_custom_index_op_via_interface.dart.strong.expect

 library test;
 import self as self;
 import "dart:core" as core;
 
 class I extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   operator [](core::int index) → core::bool
     return true;
 }
 abstract class C extends core::Object implements self::I {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 static method f() → dynamic {
   self::C c;
   core::bool x = c.{self::I::[]}(0);
 }
 static method main() → dynamic {}