Prevent analysis exceptions from crashing the completion engine.

pkg/analysis_server/lib/src/domain_completion.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library domain.completion;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analysis_server/src/constants.dart';
@@ skipping 139 matching lines @@
     if (source == null) {
       performance.complete('priorityChanged caching: no source');
       return;
     }
     performance.source = source;
     AnalysisContext context = server.getAnalysisContextForSource(source);
     if (context != null) {
       String computeTag = 'computeCache';
       performance.logStartTime(computeTag);
       CompletionManager manager = completionManagerFor(context, source);
-      manager.computeCache().then((bool success) {
+      manager.computeCache().catchError((_) => false).then((bool success) {
         performance.logElapseTime(computeTag);
         performance.complete('priorityChanged caching: $success');
       });
     }
   }
 
   /**
    * Process a `completion.getSuggestions` request.
    */
   Response processRequest(Request request, [CompletionManager manager]) {
@@ skipping 99 matching lines @@
     if (_sourcesChangedSubscription != null) {
       _sourcesChangedSubscription.cancel();
       _sourcesChangedSubscription = null;
     }
     if (_manager != null) {
       _manager.dispose();
       _manager = null;
     }
   }
 }