add ContextsChangedEvent

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 82e9fb9db23ad1bf13c35157ee2729049f3633c9..33ca5de59c5f5a1bcd883ca360da6581ac1b295b 100644
--- a/pkg/analyzer/lib/src/generated/engine.dart
+++ b/pkg/analyzer/lib/src/generated/engine.dart
@@ -284,6 +284,12 @@ class AnalysisCache {
  * proposed future state, such as the state after a refactoring.
  */
 abstract class AnalysisContext {
+
+  /**
+   * An empty list of contexts.
+   */
+  static const List<AnalysisContext> EMPTY_LIST = const <AnalysisContext>[];
+
   /**
    * Add the given listener to the list of objects that are to be notified when various analysis
    * results are produced in this context.