Inserted string validation as separate task in compiler.

frog/leg/util/characters.dart

Index: frog/leg/util/characters.dart
diff --git a/frog/leg/util/characters.dart b/frog/leg/util/characters.dart
index 910dae2efc9bf064564bcb785c3b4c417e60d572..2af6f6b015cc607c6619bd44ca1dd01412189ae8 100644
--- a/frog/leg/util/characters.dart
+++ b/frog/leg/util/characters.dart
@@ -106,4 +106,17 @@ final int $CLOSE_CURLY_BRACKET = 125;
 final int $TILDE = 126;
 final int $DEL = 127;
 final int $LS = 0x2028;
-final int $PS = 0x2029;
+final int $PS = 0x2029;
+
+bool isHexDigit(int characterCode) {
+  if (characterCode <= $9) return $0 <= characterCode;
+  characterCode |= $a ^ $A;
+  return ($a <= characterCode && characterCode <= $f);
+}
+
+int hexDigitValue(int hexDigit) {
+  assert(isHexDigit(hexDigit));
+  // hexDigit is one of '0'..'9', 'A'..'F' and 'a'..'f'.
+  if (hexDigit <= $9) return hexDigit - $0;
+  return (hexDigit | ($a ^ $A)) - ($a - 10);
+}