extract named constructor suggestions from prefixed element contributor

pkg/analysis_server/lib/src/protocol_server.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 protocol.server;
 
 import 'package:analysis_server/plugin/protocol/protocol.dart';
 import 'package:analysis_server/plugin/protocol/protocol_dart.dart';
 import 'package:analysis_server/src/services/correction/fix.dart';
 import 'package:analysis_server/src/services/search/search_engine.dart'
@@ skipping 37 matching lines @@
   String file = source.fullName;
   int fileStamp = context.getModificationStamp(source);
   change.addEdit(file, fileStamp, edit);
 }
 
 String getReturnTypeString(engine.Element element) {
   if (element is engine.ExecutableElement) {
     if (element.kind == engine.ElementKind.SETTER) {
       return null;
     } else {
-      return element.returnType.toString();
+      return element.returnType?.toString();
     }
   } else if (element is engine.VariableElement) {
     engine.DartType type = element.type;
     return type != null ? type.displayName : 'dynamic';
   } else if (element is engine.FunctionTypeAliasElement) {
     return element.returnType.toString();
   } else {
     return null;
   }
 }
@@ skipping 163 matching lines @@
     if (lineInfo != null) {
       engine.LineInfo_Location offsetLocation =
           lineInfo.getLocation(range.offset);
       startLine = offsetLocation.lineNumber;
       startColumn = offsetLocation.columnNumber;
     }
   }
   return new Location(
       source.fullName, range.offset, range.length, startLine, startColumn);
 }