Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference/instantiate_to_bounds_generic2_no_bound.dart.strong.expect

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