fix completion exception - fixes #24630

pkg/analysis_server/lib/src/services/completion/suggestion_builder.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.suggestion.builder;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analysis_server/src/protocol_server.dart' as protocol;
@@ skipping 418 matching lines @@
     return result;
   }
 
   /**
    * Add suggestions for the visible members in the given interface
    */
   static void suggestionsFor(DartCompletionRequest request, DartType type,
       {bool isSuper: false, String containingMethodName: null}) {
     CompilationUnit compilationUnit =
         request.target.containingNode.getAncestor((n) => n is CompilationUnit);
-    LibraryElement library = compilationUnit.element.library;
+    CompilationUnitElement unitElem = compilationUnit.element;
+    if (unitElem == null) {
+      return;
+    }
+    LibraryElement library = unitElem.library;
     if (type is DynamicTypeImpl) {
       type = request.cache.objectClassElement.type;
     }
     if (type is InterfaceType) {
-      return new InterfaceTypeSuggestionBuilder(request)
+      new InterfaceTypeSuggestionBuilder(request)
           ._buildSuggestions(type, library, isSuper, containingMethodName);
     }
   }
 }
 
 /**
  * This class visits elements in a library and provides suggestions based upon
  * the visible members in that library. Clients should call
  * [LibraryElementSuggestionBuilder.suggestionsFor].
  */
@@ skipping 213 matching lines @@
    * or `false` if [computeFull] should be called.
    */
   bool computeFast(AstNode node);
 
   /**
    * Return a future that computes the suggestions given a fully resolved AST.
    * The future returns `true` if suggestions were added, else `false`.
    */
   Future<bool> computeFull(AstNode node);
 }