Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference/super_property_get_invoke_implicit_call.dart.outline.expect

 library test;
 import self as self;
 import "dart:core" as core;
 
 class CallableClass extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
   method call() → core::int
     ;
 }
 class C extends core::Object {
   field dynamic f;
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class D extends self::C {
-  constructor •() → void
+  default constructor •() → void
     ;
   method g() → void
     ;
 }
 static method main() → dynamic
   ;