Set 'isSyntheticDefault' for default constructors.

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

 library test;
 import self as self;
 import "dart:core" as core;
 
 class Foo extends core::Object {
   const constructor •(core::List<core::String> l) → void
     : super core::Object::•()
     ;
   const constructor named(core::List<core::String> l) → void
     : super core::Object::•()
     ;
 }
 @self::Foo::•(const <core::String>[])
 class Bar extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 @self::Foo::named(const <core::String>[])
 class Baz extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 static method main() → dynamic {}