Handle escapes in string literals.

frog/leg/scanner/scanner_task.dart

Index: frog/leg/scanner/scanner_task.dart
diff --git a/frog/leg/scanner/scanner_task.dart b/frog/leg/scanner/scanner_task.dart
index 59bd10b3b858c61a924eef3cff363a1ddf43dbab..b56c241fa9669848e6907055024616700575fe86 100644
--- a/frog/leg/scanner/scanner_task.dart
+++ b/frog/leg/scanner/scanner_task.dart
@@ -23,7 +23,16 @@ class ScannerTask extends CompilerTask {
 
   Link<Element> scanElements(CompilationUnitElement compilationUnit) {
     Script script = compilationUnit.script;
-    Token tokens = new StringScanner(script.text).tokenize();
+    Token tokens;
+    try {
+      tokens = new StringScanner(script.text).tokenize();
+    } catch (MalformedInputException e) {
+      if (e.message is int) {
+        Token token = new Token(ERROR_INFO, e.message);
+        compiler.cancel('Unexpected character', token: token);
+        throw "Unrecoverable error";
+      }
+    }
     ElementListener listener = new ElementListener(compiler, compilationUnit);
     PartialParser parser = new PartialParser(listener);
     parser.parseUnit(tokens);