Fix analysis server 'analysis.getHover' when there is no hover information.

pkg/analysis_server/lib/src/domain_analysis.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 domain.analysis;
 
 import 'dart:collection';
 
 import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analysis_server/src/constants.dart';
@@ skipping 21 matching lines @@
    * Implement the `analysis.getHover` request.
    */
   Response getAnalysisHover(Request request) {
     // prepare parameters
     String file = request.getRequiredParameter(FILE).asString();
     int offset = request.getRequiredParameter(OFFSET).asInt();
     // prepare hovers
     List<Hover> hovers = <Hover>[];
     List<CompilationUnit> units = server.getResolvedCompilationUnits(file);
     for (CompilationUnit unit in units) {
-      hovers.add(new DartUnitHoverComputer(unit, offset).compute());
+      Hover hoverInformation = new DartUnitHoverComputer(unit, offset).compute();
+      if (hoverInformation != null) {
+        hovers.add(hoverInformation);
+      }
     }
     // send response
     Response response = new Response(request.id);
     response.setResult(HOVERS, hovers);
     return response;
   }
 
   @override
   Response handleRequest(Request request) {
     try {
@@ skipping 165 matching lines @@
 
 /**
  * A description of the change to the content of a file.
  */
 class ContentChange {
   String content;
   int offset;
   int oldLength;
   int newLength;
 }