sqAttribute constant
Escaping mode for text going into single-quoted HTML attribute values.
The result should not be used as the content of an unquoted or double-quoted attribute value.
Escapes single quotes (') but not double quotes ("),
and escapes < and > characters because they are not allowed
in strict XHTML attributes
Implementation
static const HtmlEscapeMode sqAttribute =
    const HtmlEscapeMode._('attribute', true, false, true, false)