Pull the 'keyword' property into Token.

pkg/analysis_server/lib/src/services/completion/dart/completion_manager.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.manager;
 
 import 'dart:async';
 
 import 'package:analysis_server/plugin/protocol/protocol.dart';
 import 'package:analysis_server/src/provisional/completion/completion_core.dart'
@@ skipping 466 matching lines @@
         token = token.previous;
       }
       if (token != null && isKeywordOrIdentifier(token)) {
         if (token.offset <= requestOffset && requestOffset <= token.end) {
           // Replacement range for typical identifier completion
           return new ReplacementRange(token.offset, token.length);
         }
       }
       if (token is StringToken) {
         SimpleStringLiteral uri = new SimpleStringLiteral(token, token.lexeme);
-        Token previous = token.previous;
-        if (previous is KeywordToken) {
-          Keyword keyword = previous.keyword;
-          if (keyword == Keyword.IMPORT ||
-              keyword == Keyword.EXPORT ||
-              keyword == Keyword.PART) {
-            int start = uri.contentsOffset;
-            var end = uri.contentsEnd;
-            if (start <= requestOffset && requestOffset <= end) {
-              // Replacement range for import URI
-              return new ReplacementRange(start, end - start);
-            }
+        Keyword keyword = token.previous?.keyword;
+        if (keyword == Keyword.IMPORT ||
+            keyword == Keyword.EXPORT ||
+            keyword == Keyword.PART) {
+          int start = uri.contentsOffset;
+          var end = uri.contentsEnd;
+          if (start <= requestOffset && requestOffset <= end) {
+            // Replacement range for import URI
+            return new ReplacementRange(start, end - start);
           }
         }
       }
     }
     return new ReplacementRange(requestOffset, 0);
   }
 }