Prevent analysis exceptions from crashing the completion engine.

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 70808d596e99a0557eb0265b99cbcf0b0929e794..8da57abad0d9d3f66328a0da7297171823b09482 100644
--- a/pkg/analysis_server/lib/src/domain_completion.dart
+++ b/pkg/analysis_server/lib/src/domain_completion.dart
@@ -157,7 +157,7 @@ class CompletionDomainHandler implements RequestHandler {
       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');
       });