ViewportTerm constructor
ViewportTerm(])
Implementation
ViewportTerm(Object value, String text, SourceSpan? span,
[int unit = TokenKind.UNIT_LENGTH_PX])
: super(value, text, span, unit) {
assert(unit == TokenKind.UNIT_VIEWPORT_VW ||
unit == TokenKind.UNIT_VIEWPORT_VH ||
unit == TokenKind.UNIT_VIEWPORT_VMIN ||
unit == TokenKind.UNIT_VIEWPORT_VMAX);
}