unknown constant Null safety
Default escaping mode which escape all characters.
The result of such an escaping is usable both in element content and in any attribute value.
The escaping only works for elements with normal HTML content, and not for, for example, script or style element content, which require escapes matching their particular content syntax.
static const HtmlEscapeMode unknown = HtmlEscapeMode._('unknown', true, true, true, true);