Issue 28027. Move Null to the bottom in the Analyzer.

pkg/analysis_server/lib/src/services/correction/util.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.src.correction.util;
 
 import 'dart:math';
 
 import 'package:analysis_server/plugin/protocol/protocol.dart'
     show SourceChange, SourceEdit;
@@ skipping 1111 matching lines @@
         if (parametersBuffer.length != 1) {
           parametersBuffer.write(', ');
         }
         parametersBuffer.write(parameterType);
         parametersBuffer.write(' ');
         parametersBuffer.write(parameter.name);
       }
       parametersBuffer.write(')');
       return getTypeSource(type.returnType, librariesToImport);
     }
-    // BottomType
-    if (type.isBottom) {
+    // <Bottom>, Null
+    if (type.isBottom || type.isDartCoreNull) {
       return 'dynamic';
     }
     // prepare element
     Element element = type.element;
     if (element == null) {
       String source = type.toString();
       source = source.replaceAll('<dynamic>', '');
       source = source.replaceAll('<dynamic, dynamic>', '');
       return source;
     }
@@ skipping 502 matching lines @@
       _InvertedCondition expr, int newOperatorPrecedence) {
     if (expr._precedence < newOperatorPrecedence) {
       return "(${expr._source})";
     }
     return expr._source;
   }
 
   static _InvertedCondition _simple(String source) =>
       new _InvertedCondition(2147483647, source);
 }