Add ApplyChangesStatus to AnalysisContext.applyChanges() and use it in DAS.

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 9eaeecc6913eacf3b4c1c4ae4365d0008b82c74d..63ee777366c809d340cc4bc66beadb12b98af21c 100644
--- a/pkg/analyzer/lib/src/generated/engine.dart
+++ b/pkg/analyzer/lib/src/generated/engine.dart
@@ -452,7 +452,7 @@ abstract class AnalysisContext {
    * analysis results that have been invalidated by these changes will be
    * removed.
    */
-  void applyChanges(ChangeSet changeSet);
+  ApplyChangesStatus applyChanges(ChangeSet changeSet);
 
   /**
    * Return the documentation comment for the given [element] as it appears in
@@ -1616,9 +1616,9 @@ class AnalysisContextImpl implements InternalAnalysisContext {
   }
 
   @override
-  void applyChanges(ChangeSet changeSet) {
+  ApplyChangesStatus applyChanges(ChangeSet changeSet) {
     if (changeSet.isEmpty) {
-      return;
+      return new ApplyChangesStatus(false);
     }
     //
     // First, compute the list of sources that have been removed.
@@ -1658,6 +1658,7 @@ class AnalysisContextImpl implements InternalAnalysisContext {
       _sourceRemoved(source);
     }
     _onSourcesChangedController.add(new SourcesChangedEvent(changeSet));
+    return new ApplyChangesStatus(true);
   }
 
   @override
@@ -6802,6 +6803,18 @@ abstract class AnalysisTaskVisitor<E> {
 }
 
 /**
+ * The result of applying a [ChangeSet] to a [AnalysisContext].
+ */
+class ApplyChangesStatus {
+  /**
+   * Is `true` if the given [ChangeSet] caused any changes in the context.
+   */
+  final bool hasChanges;
+
+  ApplyChangesStatus(this.hasChanges);
+}
+
+/**
  * A `CachedResult` is a single analysis result that is stored in a
  * [SourceEntry].
  */