Fix null default value access (#29512).

pkg/analysis_server/lib/src/services/completion/dart/utilities.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.
 
 /**
  * A collection of utility methods used by completion contributors.
  */
 import 'package:analysis_server/protocol/protocol_generated.dart' as protocol
     show Element, ElementKind;
 import 'package:analysis_server/src/ide_options.dart';
@@ skipping 58 matching lines @@
       sb.write(defaultValue);
       ranges.addAll([offset, defaultValue.length]);
     }
   }
 
   if (options?.generateFlutterWidgetChildrenBoilerPlate == true) {
     if (element is ConstructorElement) {
       if (isFlutterWidget(element.enclosingElement)) {
         for (ParameterElement param in element.parameters) {
           if (param.name == 'children') {
-            String defaultValue = getDefaultStringParameterValue(param);
+            String defaultValue = getDefaultStringParameterValue(param) ?? '';

[Brian Wilkerson, 2017/04/28 18:20:26]

Or just fix `getDefaultStringParameterValue` to never return `null`?

[pquitslund, 2017/04/28 18:24:25]

We use `null` as a sentinel elsewhere (in arglist_contributor) where it's useful
to distinguish none from an empty string.

Might be worth consolidating approaches though...

I'll take a look a this as I'll be in the area working on Flutter `children:`
completions (https://github.com/flutter/flutter-intellij/issues/463).

             if (sb.isNotEmpty) {
               sb.write(', ');
             }
             sb.write('children: ');
             offset = sb.length;
             sb.write(defaultValue);
             ranges.addAll([offset, defaultValue.length]);
           }
         }
       }
@@ skipping 151 matching lines @@
       //TODO (danrubel) include type arguments
     }
     return name;
   } else if (type is GenericFunctionType) {
     // TODO(brianwilkerson) Implement this.
   }
   return DYNAMIC;
 }
 
 String _getDefaultValue(ParameterElement param) => 'null';