fix combinator contributor

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

 // Copyright (c) 2015, 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.dart.suggestion.builder;
 
 import 'package:analysis_server/src/protocol_server.dart' as protocol;
 import 'package:analysis_server/src/protocol_server.dart'
     hide Element, ElementKind;
 import 'package:analyzer/src/generated/element.dart';
@@ skipping 34 matching lines @@
   suggestion.element = protocol.convertElement(element);
   Element enclosingElement = element.enclosingElement;
   if (enclosingElement is ClassElement) {
     suggestion.declaringType = enclosingElement.displayName;
   }
   suggestion.returnType = getReturnTypeString(element);
   if (element is ExecutableElement && element is! PropertyAccessorElement) {
     suggestion.parameterNames = element.parameters
         .map((ParameterElement parameter) => parameter.name)
         .toList();
-    suggestion.parameterTypes = element.parameters
-        .map((ParameterElement parameter) => parameter.type.displayName)
-        .toList();
+    suggestion.parameterTypes =
+        element.parameters.map((ParameterElement parameter) {
+      DartType paramType = parameter.type;
+      // Gracefully degrade if type not resolved yet
+      return paramType != null ? paramType.displayName : 'var';
+    }).toList();
     suggestion.requiredParameterCount = element.parameters
         .where((ParameterElement parameter) =>
             parameter.parameterKind == ParameterKind.REQUIRED)
         .length;
     suggestion.hasNamedParameters = element.parameters.any(
         (ParameterElement parameter) =>
             parameter.parameterKind == ParameterKind.NAMED);
   }
   if (importForSource != null) {
     String srcPath = path.dirname(importForSource.fullName);
@@ skipping 182 matching lines @@
       CompletionSuggestionKind kind,
       LibraryElement library,
       bool typesOnly,
       bool instCreation) {
     if (library != null) {
       library.visitChildren(new LibraryElementSuggestionBuilder(
           request, kind, typesOnly, instCreation));
     }
   }
 }