fix token type of first token in converted token stream

pkg/front_end/test/scanner_test.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:front_end/src/base/errors.dart';
 import 'package:front_end/src/base/jenkins_smi_hash.dart';
 import 'package:front_end/src/scanner/errors.dart';
 import 'package:front_end/src/scanner/reader.dart';
 import 'package:front_end/src/scanner/scanner.dart';
 import 'package:front_end/src/scanner/token.dart';
@@ skipping 914 matching lines @@
   void test_star() {
     _assertToken(TokenType.STAR, "*");
   }
 
   void test_star_eq() {
     _assertToken(TokenType.STAR_EQ, "*=");
   }
 
   void test_startAndEnd() {
     Token token = _scan("a");
+    expect(token.offset, 0);
     Token previous = token.previous;
     expect(previous.next, token);
     expect(previous.previous, previous);
+    expect(previous.type, TokenType.EOF);
+    expect(previous.offset, -1);
     Token next = token.next;
     expect(next.next, next);
     expect(next.previous, token);
+    expect(next.type, TokenType.EOF);
+    expect(next.offset, token.offset + token.length);
   }
 
   void test_string_multi_double() {
     _assertToken(TokenType.STRING, "\"\"\"line1\nline2\"\"\"");
   }
 
   void test_string_multi_embeddedQuotes() {
     _assertToken(TokenType.STRING, "\"\"\"line1\n\"\"\nline2\"\"\"");
   }
 
@@ skipping 561 matching lines @@
   _TestScanner(CharacterReader reader, [this.listener]) : super(reader);
 
   @override
   void reportError(
       ScannerErrorCode errorCode, int offset, List<Object> arguments) {
     if (listener != null) {
       listener.errors.add(new TestError(offset, errorCode, arguments));
     }
   }
 }