Use null-aware operators to clean up the code

pkg/analyzer/lib/source/analysis_options_provider.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 analyzer.source.analysis_options_provider;
 
 import 'dart:core' hide Resource;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/src/generated/engine.dart';
@@ skipping 55 matching lines @@
     if (doc is YamlMap) {
       doc.nodes.forEach((k, YamlNode v) {
         var key;
         if (k is YamlScalar) {
           key = k.value;
         }
         if (key is! String) {
           throw new OptionsFormatException(
               'Bad options file format (expected String scope key, '
               'got ${k.runtimeType})',
-              k != null ? k.span : doc.span);
+              (k ?? doc).span);
         }
         if (v != null && v is! YamlNode) {
           throw new OptionsFormatException(
               'Bad options file format (expected Node value, '
               'got ${v.runtimeType}: `${v.toString()}`)',
               doc.span);
         }
         options[key] = v;
       });
     }
@@ skipping 31 matching lines @@
 /// Thrown on options format exceptions.
 class OptionsFormatException implements Exception {
   final String message;
   final SourceSpan span;
   OptionsFormatException(this.message, [this.span]);
 
   @override
   String toString() =>
       'OptionsFormatException: ${message?.toString()}, ${span?.toString()}';
 }