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)

lib/core/parser/characters.dart

@@ -98,12 +98,7 @@ const int $TILDE  = 126;
 const int $NBSP   = 160;
 
 bool isWhitespace(int code) {
-  return (code == $SPACE)
-      || (code == $CR)
-      || (code == $LF)
-      || (code == $NBSP)
-      || (code == $TAB)
-      || (code == $VTAB);
+  return (code >= $TAB && code <= $SPACE) || (code == $NBSP);
 }
 
 bool isIdentifierStart(int code) {
@@ -134,6 +129,7 @@ bool isExponentSign(int code) {
 }
 
 int unescape(int code) {
+  if (code & 1 == 1 || code < $f || code > $v) return code;
   Map<int, int> escapes = const {
       $n: $LF,
       $f: $FF,