Set 'isSyntheticDefault' for default constructors.

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

 library test;
 import self as self;
 import "dart:core" as core;
 
 class A<T extends core::Object> extends core::Object {
   field core::List<self::A::T> x = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   set y(core::List<self::A::T> value) → void {}
 }
 static method main() → dynamic {
   self::A<core::int> a_int = new self::A::•<core::int>();
   self::A<core::Object> a_object = new self::A::•<core::Object>();
   self::A<dynamic> a_dynamic = new self::A::•<dynamic>();
   dynamic x_int = a_int.x = <dynamic>[0];
   dynamic y_int = a_int.y = <dynamic>[0];
   dynamic x_object = a_object.x = <dynamic>[0];
   dynamic y_object = a_object.y = <dynamic>[0];
   dynamic x_dynamic = a_dynamic.x = <dynamic>[0];
   dynamic y_dynamic = a_dynamic.y = <dynamic>[0];
   dynamic x_int_explicit = a_int.x = <core::int>[0];
   dynamic y_int_explicit = a_int.y = <core::int>[0];
   dynamic x_object_explicit = a_object.x = <core::int>[0];
   dynamic y_object_explicit = a_object.y = <core::int>[0];
   dynamic x_dynamic_explicit = a_dynamic.x = <core::int>[0];
   dynamic y_dynamic_explicit = a_dynamic.y = <core::int>[0];
   core::List<core::int> x_int_downward = a_int.x = <dynamic>[0];
   core::List<core::int> y_int_downward = a_int.y = <dynamic>[0];
   core::List<core::int> x_object_downward = a_object.x = <dynamic>[0];
   core::List<core::int> y_object_downward = a_object.y = <dynamic>[0];
   core::List<core::int> x_dynamic_downward = a_dynamic.x = <dynamic>[0];
   core::List<core::int> y_dynamic_downward = a_dynamic.y = <dynamic>[0];
 }