LengthTerm constructor
Implementation
LengthTerm(super.value, super.text, super.span,
[super.unit = TokenKind.UNIT_LENGTH_PX]) {
assert(unit == TokenKind.UNIT_LENGTH_PX ||
unit == TokenKind.UNIT_LENGTH_CM ||
unit == TokenKind.UNIT_LENGTH_MM ||
unit == TokenKind.UNIT_LENGTH_IN ||
unit == TokenKind.UNIT_LENGTH_PT ||
unit == TokenKind.UNIT_LENGTH_PC);
}