AngleTerm constructor
Implementation
AngleTerm(super.value, super.text, super.span,
[super.unit = TokenKind.UNIT_LENGTH_PX]) {
assert(unit == TokenKind.UNIT_ANGLE_DEG ||
unit == TokenKind.UNIT_ANGLE_RAD ||
unit == TokenKind.UNIT_ANGLE_GRAD ||
unit == TokenKind.UNIT_ANGLE_TURN);
}