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 1172 matching lines @@
 
   void test_unclosedPairInInterpolation() {
     ErrorListener listener = new ErrorListener();
     scanWithListener("'\${(}'", listener);
   }
 
   void test_unmatched_openers() {
     var openBrace = _scan('{[(') as BeginToken;
     var openBracket = openBrace.next as BeginToken;
     var openParen = openBracket.next as BeginToken;
+    expect(openBrace.previous.type, TokenType.EOF);

[Paul Berry, 2017/02/16 22:07:54]

This seems like a weird place to test this functionality.  How about creating a
new test called something like "test_start_token"?

Also, while we're at it, we should test that the offset of the start token is
correct (currently it's -1 for analyzer's scanner, and 0 for the token stream
from ToAnalyzerTokenStream).

[danrubel, 2017/02/21 14:49:27]

Good points. Found an already existing test_startAndEnd test and enhanced that
one instead.

     expect(openParen.next.type, TokenType.EOF);
     expect(openBrace.endToken, isNull);
     expect(openBracket.endToken, isNull);
     expect(openParen.endToken, isNull);
   }
 
   void _assertComment(TokenType commentType, String source,
       {bool genericMethodComments: false}) {
     //
     // Test without a trailing end-of-line marker
@@ skipping 309 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));
     }
   }
 }