perf(parser): Faster character tests.

Performance
===========

lexer_perf:

Dart VM - Old:

ident: => 1,182,988 ops/sec (1 us) stdev(0.1318)
ident-path: => 1,805,782 ops/sec (1 us) stdev(0.26363)
num: => 1,269,793 ops/sec (1 us) stdev(0.17873)
num-double: => 760,554 ops/sec (1 us) stdev(0.13409)
string: => 1,968,371 ops/sec (1 us) stdev(0.20383)
string-escapes: => 607,930 ops/sec (2 us) stdev(0.08734)

Dart VM - New:

ident: => 2,068,501 ops/sec (0 us) stdev(0.05876)
ident-path: => 3,234,291 ops/sec (0 us) stdev(0.09304)
num: => 1,972,071 ops/sec (1 us) stdev(0.16793)
num-double: => 953,725 ops/sec (1 us) stdev(0.022)
string: => 2,108,910 ops/sec (0 us) stdev(0.08699)
string-escapes: => 712,866 ops/sec (1 us) stdev(0.02661)