Handle escapes in string literals.

frog/leg/scanner/scanner.dart

Index: frog/leg/scanner/scanner.dart
diff --git a/frog/leg/scanner/scanner.dart b/frog/leg/scanner/scanner.dart
index bfce2342cb9597ab318b32124ef838c973f29a26..d55c53a179c20c487ebdf4d024968509a9d64218 100644
--- a/frog/leg/scanner/scanner.dart
+++ b/frog/leg/scanner/scanner.dart
@@ -629,6 +629,12 @@ class AbstractScanner<T> implements Scanner {
     }
   }
 
+  static bool isHexDigit(int character) {
+    if ($0 <= character && character <= $9) return true;
+    character |= 0x20;
+    return ($a <= character && character <= $f);
+  }
+
   int tokenizeSingleLineString(int next, int q1, int start) {
     while (next !== $EOF) {
       if (next === q1) {