Improve parse speed by switching from regexps to simple string operations.
Improve parse speed by switching from regexps to simple string operations.

50k short flags
before: 11ms, 11ms, 13ms, 11ms, 17ms
after: 6ms, 5ms, 6ms, 5ms, 8ms

50k abbreviations
before: 97ms, 114ms, 120ms, 111ms, 107ms
after: 39ms, 37ms, 51ms, 36ms, 36ms

50k long flags
before: 27ms, 26ms, 24ms, 24ms, 28ms
after: 15ms, 19ms, 19ms, 15ms, 15ms

50k long options with =
before: 27ms, 26ms, 27ms, 24ms, 26ms
after: 15ms, 19ms, 19ms, 15ms, 15ms

dart-lang/args