Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/bad_store.dart.direct.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class Foo extends core::Object {
   field dynamic field = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 static method identity(dynamic x) → dynamic
   return x;
 static method use(dynamic x) → void {}
 static method main(core::List<core::String> args) → dynamic {
   dynamic foo = self::identity(new self::Foo::•());
   if(args.length.>(1)) {
     foo.field = "string";
     dynamic first = foo.field;
     self::use(first);
     foo.noField = "string";
     dynamic second = foo.noField;
     self::use(second);
   }
 }