Handle exception while getting modification stamp

pkg/analyzer/lib/src/generated/source.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.
 
 // This code was auto-generated, is not intended to be edited, and is subject to
 // significant change. Please see the README file for more information.
 
 library engine.source;
 
 import "dart:math" as math;
@@ skipping 421 matching lines @@
   int get hashCode;
 
   /**
    * Return `true` if this source is in one of the system libraries.
    *
    * @return `true` if this is in a system library
    */
   bool get isInSystemLibrary;
 
   /**
-   * Return the modification stamp for this source. A modification stamp is a non-negative integer
-   * with the property that if the contents of the source have not been modified since the last time
-   * the modification stamp was accessed then the same value will be returned, but if the contents
-   * of the source have been modified one or more times (even if the net change is zero) the stamps
-   * will be different.
+   * Return the modification stamp for this source, or a negative value if the
+   * source does not exist. A modification stamp is a non-negative integer with
+   * the property that if the contents of the source have not been modified
+   * since the last time the modification stamp was accessed then the same value
+   * will be returned, but if the contents of the source have been modified one
+   * or more times (even if the net change is zero) the stamps will be different.
    *
    * Clients should consider using the the method
-   * [AnalysisContext.getModificationStamp] because contexts can have local overrides
-   * of the content of a source that the source is not aware of.
-   *
-   * @return the modification stamp for this source
+   * [AnalysisContext.getModificationStamp] because contexts can have local
+   * overrides of the content of a source that the source is not aware of.
    */
   int get modificationStamp;
 
   /**
    * Return a short version of the name that can be displayed to the user to denote this source. For
    * example, for a source representing a file this would typically be the name of the file.
    *
    * @return a name that can be displayed to the user to denote this source
    */
   String get shortName;
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   /**
    * Return an absolute URI that represents the given source, or `null` if a valid URI cannot
    * be computed.
    *
    * @param source the source to get URI for
    * @return the absolute URI representing the given source
    */
   Uri restoreAbsolute(Source source) => null;
 }