fix named constructor completion

pkg/analysis_server/lib/src/services/completion/invocation_computer.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 services.completion.computer.dart.invocation;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/services/completion/dart_completion_manager.dart';
 import 'package:analysis_server/src/services/completion/suggestion_builder.dart';
@@ skipping 62 matching lines @@
 /**
  * An [AstNode] vistor for determining which suggestion builder
  * should be used to build invocation/access suggestions.
  */
 class _InvocationAstVisitor extends GeneralizingAstVisitor<SuggestionBuilder> {
   final DartCompletionRequest request;
 
   _InvocationAstVisitor(this.request);
 
   @override
+  visitConstructorName(ConstructorName node) {
+    // some PrefixedIdentifier nodes are transformed into
+    // ConstructorName nodes during the resolution process.
+    Token period = node.period;
+    if (period != null && request.offset > period.offset) {
+      TypeName type = node.type;
+      if (type != null) {
+        SimpleIdentifier prefix = type.name;
+        if (prefix != null) {
+          return new _PrefixedIdentifierSuggestionBuilder(request);
+        }
+      }
+    }
+    return null;
+  }
+
+  @override
   SuggestionBuilder visitMethodInvocation(MethodInvocation node) {
     Token period = node.period;
     if (period == null || period.offset < request.offset) {
       return new _ExpressionSuggestionBuilder(request);
     }
     return null;
   }
 
   @override
   SuggestionBuilder visitNode(AstNode node) {
     return null;
   }
 
   @override
   SuggestionBuilder visitPrefixedIdentifier(PrefixedIdentifier node) {
     // some PrefixedIdentifier nodes are transformed into
     // ConstructorName nodes during the resolution process.
     Token period = node.period;
-    if (request.offset > period.offset) {
+    if (period != null && request.offset > period.offset) {
       SimpleIdentifier prefix = node.prefix;
       if (prefix != null) {
         return new _PrefixedIdentifierSuggestionBuilder(request);
       }
     }
     return null;
   }
 
   @override
   SuggestionBuilder visitPropertyAccess(PropertyAccess node) {
@@ skipping 108 matching lines @@
 
   @override
   Future<bool> visitVariableElement(VariableElement element) {
     DartType type = element.type;
     if (type != null) {
       ClassElementSuggestionBuilder.suggestionsFor(request, type.element);
     }
     return new Future.value(true);
   }
 }