Remove unnecessary dart:scalarlist import.

Signed-off-by: John McCutchan <john@johnmccutchan.com>
Rename negate_ to negate.

Signed-off-by: John McCutchan <john@johnmccutchan.com>
Fix import

Signed-off-by: John McCutchan <john@johnmccutchan.com>
Test negate method.

Signed-off-by: John McCutchan <john@johnmccutchan.com>