Handle escapes in string literals.

frog/leg/scanner/token.dart

Index: frog/leg/scanner/token.dart
diff --git a/frog/leg/scanner/token.dart b/frog/leg/scanner/token.dart
index cc3c3285c05720e88cecd56eb66012bca8aba389..3ce6830118643ae2f47cbc6971c56e401c7401d4 100644
--- a/frog/leg/scanner/token.dart
+++ b/frog/leg/scanner/token.dart
@@ -128,6 +128,10 @@ interface SourceString extends Hashable default StringWrapper {
   int get length();
 
   void printOn(StringBuffer sb);
+  void printSubstringOn(StringBuffer sb, int from, int to);
+
+  int charCodeAt(int i);
+  String operator[](int index);
 
   String get stringValue();
 }
@@ -149,6 +153,14 @@ class StringWrapper implements SourceString {
     sb.add(stringValue);
   }
 
+  void printSubstringOn(StringBuffer sb, int from, int to) {
+    if (from >= to) return;
+    sb.add(stringValue.substring(from, to));
+  }
+
+  int charCodeAt(int index) => stringValue.charCodeAt(index);
+  String operator[](int index) => stringValue[index];
+
   String toString() => stringValue;
 }