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

pkg/analysis_server/lib/src/analysis_server.dart

Index: pkg/analysis_server/lib/src/analysis_server.dart
diff --git a/pkg/analysis_server/lib/src/analysis_server.dart b/pkg/analysis_server/lib/src/analysis_server.dart
index 088d2a60dfa4efc89a7580df43b82d6a9103d9e3..4d3af67d9f1fca1ea1edfbbe0d2a934d36694260 100644
--- a/pkg/analysis_server/lib/src/analysis_server.dart
+++ b/pkg/analysis_server/lib/src/analysis_server.dart
@@ -1465,8 +1465,10 @@ class ServerContextManagerCallbacks extends ContextManagerCallbacks {
   void applyChangesToContext(Folder contextFolder, ChangeSet changeSet) {
     AnalysisContext context = analysisServer.folderMap[contextFolder];
     if (context != null) {
-      context.applyChanges(changeSet);
-      analysisServer.schedulePerformAnalysisOperation(context);
+      ApplyChangesStatus changesStatus = context.applyChanges(changeSet);
+      if (changesStatus.hasChanges) {
+        analysisServer.schedulePerformAnalysisOperation(context);
+      }
       List<String> flushedFiles = new List<String>();
       for (Source source in changeSet.removedSources) {
         flushedFiles.add(source.fullName);