Set 'isSyntheticDefault' for default constructors.

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

 library test;
 import self as self;
 import "dart:core" as core;
 
 typedef F<T extends core::Object> = () → void;
 class Foo extends core::Object {
   const constructor •(core::List<core::String> l) → void
     : super core::Object::•()
     ;
 }
 class C<T extends core::Object> extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class D extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   method m<T extends core::Object>() → void {}
 }
 static method f<T extends core::Object>() → void {}
 static method main() → dynamic {}