invoke computeCache when priority sources change

pkg/analysis_server/lib/src/domain_completion.dart

Index: pkg/analysis_server/lib/src/domain_completion.dart
diff --git a/pkg/analysis_server/lib/src/domain_completion.dart b/pkg/analysis_server/lib/src/domain_completion.dart
index a3ea628c4f48ee9cd4b807cfe9bf36b49290b313..dc6cc9e183466acda31b8841250ecaf50a6a4fdd 100644
--- a/pkg/analysis_server/lib/src/domain_completion.dart
+++ b/pkg/analysis_server/lib/src/domain_completion.dart
@@ -53,6 +53,7 @@ class CompletionDomainHandler implements RequestHandler {
    */
   CompletionDomainHandler(this.server) {
     server.onContextsChanged.listen(contextsChanged);
+    server.onPriorityChange.listen(priorityChanged);
   }
 
   /**
@@ -114,6 +115,22 @@ class CompletionDomainHandler implements RequestHandler {
   }
 
   /**
+   * If the set the priority files has changed, then pre-cache completion
+   * information related to the first priority file.
+   */
+  void priorityChanged(PriorityChangeEvent event) {
+    Source source = event.firstSource;
+    if (source == null) {
+      return;
+    }
+    AnalysisContext context = server.getAnalysisContextForSource(source);
+    if (context == null) {
+      return;
+    }
+    completionManagerFor(context, source).computeCache();
+  }
+
+  /**
    * Process a `completion.getSuggestions` request.
    */
   Response processRequest(Request request) {