Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/cycles.dart.direct.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class A extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class B extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class C extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class D extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 const field dynamic #errors = const <dynamic>["pkg/front_end/testcases/cycles.dart:7:7: Error: 'B' is a supertype of itself via 'A', 'C', 'B'.\nclass B extends A {}\n      ^"]/* from null */;
 static method main() → dynamic {
   core::print(new self::A::•());
   core::print(new self::B::•());
   core::print(new self::C::•());
   core::print(new self::D::•());
 }