filter undesired suggestions from prefix expression

pkg/analysis_server/lib/src/services/completion/imported_computer.dart

Index: pkg/analysis_server/lib/src/services/completion/imported_computer.dart
diff --git a/pkg/analysis_server/lib/src/services/completion/imported_computer.dart b/pkg/analysis_server/lib/src/services/completion/imported_computer.dart
index f8e19a96f0fdbf44ac2256b6c3e3b42dcf0265de..8a754f9113633f17f0d539f5b4f78dbfcb6a4287 100644
--- a/pkg/analysis_server/lib/src/services/completion/imported_computer.dart
+++ b/pkg/analysis_server/lib/src/services/completion/imported_computer.dart
@@ -66,6 +66,11 @@ class _ImportedVisitor extends GeneralizingAstVisitor<Future<bool>> {
     if (parent is ExpressionStatement) {
       return _addImportedElementSuggestions();
     }
+    if (parent is PrefixedIdentifier) {
+      if (request.offset <= parent.prefix.end) {
+        return _addImportedElementSuggestions();
+      }
+    }
     return new Future.value(false);
   }