Set 'isSyntheticDefault' for default constructors.

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

 library test;
 import self as self;
 import "dart:core" as core;
 
 class A<T extends core::Object> extends core::Object {
   constructor •(self::B<core::List<self::A::T>> b) → void
     : super core::Object::•()
     ;
 }
 class B<T extends core::Object> extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 static method main() → dynamic {
   dynamic x = new self::A::•<dynamic>(new self::B::•<dynamic>());
 }