Handle escapes in string literals.

frog/leg/scanner/string_scanner.dart

Index: frog/leg/scanner/string_scanner.dart
diff --git a/frog/leg/scanner/string_scanner.dart b/frog/leg/scanner/string_scanner.dart
index 58efaa0990750693345a1e61b92fe385f053021b..979c37ec1e6300be7292b6c45557a57567aa30c0 100644
--- a/frog/leg/scanner/string_scanner.dart
+++ b/frog/leg/scanner/string_scanner.dart
@@ -53,6 +53,15 @@ class SubstringWrapper implements SourceString {
     sb.add(this);
   }
 
+  void printSubstringOn(StringBuffer sb, int from, int to) {
+    if (from >= to) return;
+    sb.add(internalString.substring(begin + from, begin + to));
+  }
+
+  int charCodeAt(int index) => internalString.charCodeAt(begin + index);
+
+  String operator[](int index) => internalString[begin + index];
+
   String toString() => internalString.substring(begin, end);
 
   String get stringValue() => toString();