Set 'isSyntheticDefault' for default constructors.

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

 library test;
 import self as self;
 import "dart:core" as core;
 
 class C extends core::Object {
   field core::int x = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 static method f(self::C c) → void {
   core::int x = let final self::C #t1 = c in #t1.==(null) ?{core::int} null : #t1.{self::C::x};
   let final self::C #t2 = c in #t2.==(null) ?{core::int} null : #t2.{self::C::x};
 }
 static method main() → dynamic {}