Handle exception while getting modification stamp

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

Index: pkg/analyzer/lib/src/generated/engine.dart
diff --git a/pkg/analyzer/lib/src/generated/engine.dart b/pkg/analyzer/lib/src/generated/engine.dart
index 75b37af269f4c967886c53a61bc5194f4172f6b1..abac83f3badd07d75f466e6cb2b971ffc1f11dd6 100644
--- a/pkg/analyzer/lib/src/generated/engine.dart
+++ b/pkg/analyzer/lib/src/generated/engine.dart
@@ -715,17 +715,16 @@ abstract class AnalysisContext {
   LineInfo getLineInfo(Source source);
 
   /**
-   * Return the modification stamp for the given 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.
-   *
-   * This method should be used rather than the method [Source.getModificationStamp] because
-   * contexts can have local overrides of the content of a source that the source is not aware of.
-   *
-   * @param source the source whose modification stamp is to be returned
-   * @return the modification stamp for the source
+   * Return the modification stamp for the [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.
+   *
+   * This method should be used rather than the method
+   * [Source.getModificationStamp] because contexts can have local overrides of
+   * the content of a source that the source is not aware of.
    */
   int getModificationStamp(Source source);