Set 'isSyntheticDefault' for default constructors.

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

 library test;
 import self as self;
 import "dart:core" as core;
 
 class A extends core::Object {
   field self::B<core::int> b = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class B<T extends core::Object> extends core::Object {
   constructor •(self::B::T x) → void
     : super core::Object::•()
     ;
 }
 static field core::List<self::B<core::int>> t3 = <self::B<core::int>>[new self::B::•<core::int>(3)];
 static method main() → dynamic {}