Add AnalysisContext.onResultInvalidated(descriptor).

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 57d35d91212d749e4133189ddc1390cfaf43d1d1..02621a2cc06043e9f8bdd626bc379617090250db 100644
--- a/pkg/analyzer/lib/src/generated/engine.dart
+++ b/pkg/analyzer/lib/src/generated/engine.dart
@@ -520,6 +520,13 @@ abstract class AnalysisContext {
   Stream<ComputedResult> onResultComputed(ResultDescriptor descriptor);
 
   /**
+   * Return the stream that is notified when a result with the given
+   * [descriptor] is invalidated.
+   */
+  Stream<InvalidatedResultEvent> onResultInvalidated(
+      ResultDescriptor descriptor);
+
+  /**
    * Parse the content of the given [source] to produce an AST structure. The
    * resulting AST structure may or may not be resolved, and may have a slightly
    * different structure depending upon whether it is resolved.
@@ -1865,7 +1872,7 @@ class ComputedResult<V> {
   ComputedResult(this.context, this.descriptor, this.target, this.value);
 
   @override
-  String toString() => '$descriptor of $target in $context';
+  String toString() => 'Computed $descriptor of $target in $context';
 }
 
 /**
@@ -2059,6 +2066,38 @@ abstract class InternalAnalysisContext implements AnalysisContext {
 }
 
 /**
+ * [InvalidatedResultEvent] describes an invalidated analysis result.
+ */
+class InvalidatedResultEvent<V> {

[Brian Wilkerson, 2016/03/08 18:36:36]

Given that it's now identical to ComputedResult (except for toString), I have to
wonder whether there's any value in adding a new class.

I'd recommend generalizing this:
- rename this class to ResultChange (or ResultChangedEvent),
- rename the method to onResultChanged, and
- add two getters wasComputed and wasInvalidated (with a private field).

It would be a breaking change to remove ComputedResult, but we could deprecate
both it and onResultComputed.

+  /**
+   * The context in which the result was invalidated.
+   */
+  final AnalysisContext context;
+
+  /**
+   * The target for which the result was invalidated.
+   */
+  final AnalysisTarget target;
+
+  /**
+   * The descriptor of the result which was invalidated.
+   */
+  final ResultDescriptor<V> descriptor;
+
+  /**
+   * The value of the result before it was invalidated, may be the default
+   * value if the result was flushed.
+   */
+  final V value;
+
+  InvalidatedResultEvent(
+      this.context, this.target, this.descriptor, this.value);
+
+  @override
+  String toString() => 'Invalidated $descriptor of $target';
+}
+
+/**
  * An object that can be used to receive information about errors within the
  * analysis engine. Implementations usually write this information to a file,
  * but can also record the information for later use (such as during testing) or