Fix incremental scanner's computation of scanEnd.

pkg/analyzer/lib/src/generated/incremental_scanner.dart

 // Copyright (c) 2014, 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.
 
 library engine.incremental_scanner;
 
 import "dart:math" as math;
 
 import 'error.dart';
 import 'scanner.dart';
@@ skipping 118 matching lines @@
       oldRightToken = stream;
     } else {
       oldLast = stream.previous;
       oldRightToken = stream;
     }
     //
     // Compute the range of characters that are known to need to be rescanned.
     // If the index is within an existing token, then we need to start at the
     // beginning of the token.
     //
-    int scanStart = math.max(oldFirst.previous.end, 0);
-    int scanEnd = oldLast.end + delta;
+    int scanStart = math.max(oldLeftToken.end, 0);
+    int scanEnd = oldRightToken.offset + delta;
     //
     // Rescan the characters that need to be rescanned.
     //
     Token replacementStart = _scanRange(scanStart, scanEnd);
     oldLeftToken.setNext(replacementStart);
     Token replacementEnd = _findEof(replacementStart).previous;
     replacementEnd.setNext(stream);
     //
     // Apply the delta to the tokens after the last new token.
     //
@@ skipping 84 matching lines @@
       while (comment != null) {
         comment.offset += delta;
         comment = comment.next;
       }
       token = token.next;
     }
     _tokenMap.put(token, token);
     token.offset += delta;
   }
 }