Consider fontifying operators when being declared.

[bradyt]

Operator method declarations are not being highlighted in the following.

class DateTime implements Comparable<DateTime> {
  bool operator ==(other) {
    if (!(other is DateTime)) return false;
    return (_value == other._value && isUtc == other.isUtc);
  }
}

It's not obvious to me what fontification would be idiomatic to Dart and Emacs. For example, we might fontify operators elsewhere in general.

[bradyt]

Fixed at https://github.com/bradyt/dart-mode/commit/5989dca60f5c03a74d0eb4cd40a22a735a9ffe9f.