Make source be a target

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

Index: pkg/analyzer/lib/src/generated/source.dart
diff --git a/pkg/analyzer/lib/src/generated/source.dart b/pkg/analyzer/lib/src/generated/source.dart
index b3c117831733a3801846e986a313da33a83d5f47..2a06746b459a078d21db0bfb5e46958279563283 100644
--- a/pkg/analyzer/lib/src/generated/source.dart
+++ b/pkg/analyzer/lib/src/generated/source.dart
@@ -12,6 +12,7 @@ import 'dart:collection';
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/source/package_map_resolver.dart';
+import 'package:analyzer/task/model.dart';
 
 import 'engine.dart';
 import 'java_core.dart';
@@ -309,7 +310,7 @@ class LocalSourcePredicate_TRUE implements LocalSourcePredicate {
 /**
  * An implementation of an non-existing [Source].
  */
-class NonExistingSource implements Source {
+class NonExistingSource extends Source {
   final String _name;
 
   final UriKind uriKind;
@@ -382,7 +383,7 @@ class NonExistingSource implements Source {
  * retained so that if those files are created at a later date the long-lived sources representing
  * those files will know that they now exist.
  */
-abstract class Source {
+abstract class Source implements AnalysisTarget {
   /**
    * An empty list of sources.
    */
@@ -409,6 +410,9 @@ abstract class Source {
    */
   String get encoding;
 
+  @override
+  Source get source => this;
+
   /**
    * Return the full (long) 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 absolute path