Various scanner fixes:

dart/frog/leg/tools/mini_parser.dart

Index: dart/frog/leg/tools/mini_parser.dart
diff --git a/dart/frog/leg/tools/mini_parser.dart b/dart/frog/leg/tools/mini_parser.dart
index 7f6c4997e431b698427feb025d56379d3368b3a6..85096e68344974d4a78476f702bd58406a586726 100644
--- a/dart/frog/leg/tools/mini_parser.dart
+++ b/dart/frog/leg/tools/mini_parser.dart
@@ -19,51 +19,87 @@ void main() {
   stopwatch.start();
   List<String> filenames = new Options().arguments;
   bool diet = false;
+  bool throwOnError = false;
+  bool scanOnly = false;
   int charCount = 0;
   for (String filename in filenames) {
     if (filename == "--diet") {
       diet = true;
       continue;
     }
+    if (filename == "--throw") {
+      throwOnError = true;
+      continue;
+    }
+    if (filename == "--scan-only") {
+      scanOnly = true;
+      continue;
+    }
     String source = new String.fromCharCodes(read(filename));
     charCount += source.length;
     SourceFile file = new SourceFile(filename, source);
     Listener listener = new MyListener(file);
     Parser parser = diet ? new PartialParser(listener) : new Parser(listener);
     try {
-      parser.parseUnit(scan(file.text));
+      Token token = scan(file);
+      if (!scanOnly) parser.parseUnit(token);
     } catch (ParserError ex) {
-      print(ex);
+      if (throwOnError) {
+        throw;
+      } else {
+        print(ex);
+      }
     }
   }
   stopwatch.stop();
   int kb = (charCount/1024).round().toInt();
   String stats =
-      '$classCount classes (${kb}Kb) in ${stopwatch.elapsedInMs()}ms.';
+      '$classCount classes (${kb}Kb) in ${stopwatch.elapsedInMs()}ms';
+  if (errorCount != 0) {
+    stats += ' with $errorCount errors';
+  }
   if (diet) {
-    print('Diet parsed $stats');
+    print('Diet parsed $stats.');
   } else {
-    print('Parsed $stats');
+    print('Parsed $stats.');
   }
 }
 
-Token scan(String source) => new StringScanner(source).tokenize();
+Token scan(SourceFile source) {
+  Scanner scanner = new StringScanner(source.text);
+  try {
+    return scanner.tokenize();
+  } catch (MalformedInputException ex) {
+    var message = ex.message;
+    if (message is !String) message = "unexpected character";
+    Token fakeToken = new Token($QUESTION, scanner.charOffset);
+    new MyListener(source).error(message, fakeToken);
+    throw;
+  }
+}
 
 List<int> read(String filename) {
   File file = new File(filename);
   file.openSync();
+  bool threw = true;
   try {
     int size = file.lengthSync();
     List<int> bytes = new List<int>(size + 1);
     file.readListSync(bytes, 0, size);
     bytes[size] = $EOF;
+    threw = false;
     return bytes;
   } finally {
-    file.closeSync();
+    try {
+      file.closeSync();
+    } catch (var ex) {
+      if (!threw) throw;
+    }
   }
 }
 
 int classCount = 0;
+int errorCount = 0;
 
 class MyListener extends Listener {
   final SourceFile file;
@@ -75,6 +111,7 @@ class MyListener extends Listener {
   }
 
   void error(String message, Token token) {
+    ++errorCount;
     if (token !== null) {
       String tokenString = token.toString();
       int begin = token.charOffset;