Guard against an AST node with no end token (issue 30089)

pkg/analyzer_plugin/lib/src/utilities/completion/completion_target.dart

 // Copyright (c) 2017, 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.
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_ast_factory.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/src/dart/ast/token.dart';
@@ skipping 433 matching lines @@
     // (e.g. inside a literal string or inside a comment) are evaluated within
     // the context of the token itself.
     return offset <= node.offset;
   }
 
   /**
    * Determine whether [token] could possibly be the [entity] for a
    * [CompletionTarget] associated with the given [offset].
    */
   static bool _isCandidateToken(Token token, int offset) {
+    if (token == null) {
+      return false;
+    }
     // A token is considered a candidate entity if the cursor offset is (a)
     // before the start of the token, (b) within the token, (c) at the end of
     // the token and the token is a keyword or identifier, or (d) at the
     // location of the token and the token is zero length.
     if (offset < token.end) {
       return true;
     } else if (offset == token.end) {
       return token.type.isKeyword ||
           token.type == TokenType.IDENTIFIER ||
           token.length == 0;
@@ skipping 30 matching lines @@
               orElse: () => null);
           paramType = param?.type;
         }
       } else {
         paramType = param.type;
       }
     }
     return paramType is FunctionType || paramType is FunctionTypeAlias;
   }
 }