Gauge constructor Null safety
Implementation
Gauge(String name, String description, this.min, this.max)
: _value = min,
super(name, description) {
// TODO: When NNBD is complete, delete the following two lines.
ArgumentError.checkNotNull(min, 'min');
ArgumentError.checkNotNull(max, 'max');
if (!(min < max)) throw new ArgumentError('min must be less than max');
}