fix code completion import dart exception

pkg/analysis_server/lib/src/services/completion/imported_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.toplevel;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/protocol.dart' hide Element;
 import 'package:analysis_server/src/services/completion/dart_completion_manager.dart';
@@ skipping 91 matching lines @@
           new Map<LibraryElement, Set<String>>();
 
       // Exclude elements from the local library
       // as they will be included by the LocalComputer
       excludedLibs.add(request.unit.element.library);
 
       // Build the set of visible and excluded libraries
       // and the list of names that should be shown or hidden
       request.unit.directives.forEach((Directive directive) {
         if (directive is ImportDirective) {
-          LibraryElement lib = directive.element.importedLibrary;
-          if (directive.prefix == null) {
-            visibleLibs.add(lib);
-            directive.combinators.forEach((Combinator combinator) {
-              if (combinator is ShowCombinator) {
-                showNames[lib] =
-                    combinator.shownNames.map((SimpleIdentifier id) => id.name).toSet();
-              } else if (combinator is HideCombinator) {
-                hideNames[lib] =
-                    combinator.hiddenNames.map((SimpleIdentifier id) => id.name).toSet();
-              }
-            });
-          } else {
-            excludedLibs.add(lib);
+          ImportElement element = directive.element;
+          if (element != null) {
+            LibraryElement lib = element.importedLibrary;
+            if (directive.prefix == null) {
+              visibleLibs.add(lib);
+              directive.combinators.forEach((Combinator combinator) {
+                if (combinator is ShowCombinator) {
+                  showNames[lib] =
+                      combinator.shownNames.map((SimpleIdentifier id) => id.name).toSet();
+                } else if (combinator is HideCombinator) {
+                  hideNames[lib] =
+                      combinator.hiddenNames.map((SimpleIdentifier id) => id.name).toSet();
+                }
+              });
+            } else {
+              excludedLibs.add(lib);
+            }
           }
         }
       });
 
       // Compute the set of possible classes, functions, and top level variables
       matches.forEach((SearchMatch match) {
         if (match.kind == MatchKind.DECLARATION) {
           Element element = match.element;
           LibraryElement lib = element.library;
           if (element.isPublic && !excludedLibs.contains(lib)) {
@@ skipping 37 matching lines @@
 
               request.suggestions.add(suggestion);
             }
           }
         }
       });
       return true;
     });
   }
 }