Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference_new/infer_assign_to_ref.dart.strong.expect

 library test;
 import self as self;
 import "dart:core" as core;
 
 class A extends core::Object {
   field core::int f = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 static field self::A a = new self::A::•();
 static field core::int b = self::a.{self::A::f} = 1;
 static field core::int c = 0;
 static field core::int d = self::c = 1;
 static method main() → dynamic {
   self::a;
   self::b;
   self::c;
   self::d;
 }