Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference_new/infer_use_of_void.dart.strong.expect

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