Handle context == null in the analysis.getHover request.

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 23 matching lines @@
    */
   Response getAnalysisHover(Request request) {
     // prepare parameters
     String file = request.getRequiredParameter(FILE).asString();
     int offset = request.getRequiredParameter(OFFSET).asInt();
     // prepare hovers
     List<Map<String, Object>> hovers = <Map<String, Object>>[];
     {
       Source source = server.getSource(file);
       AnalysisContext context = server.getAnalysisContext(file);
-      List<Source> librarySources = context.getLibrariesContaining(source);
-      for (Source librarySource in librarySources) {
-        CompilationUnit unit = context.resolveCompilationUnit2(source,
-            librarySource);
-        hovers.add(new DartUnitHoverComputer(unit, offset).compute());
+      if (context != null) {
+        List<Source> librarySources = context.getLibrariesContaining(source);
+        for (Source librarySource in librarySources) {
+          CompilationUnit unit = context.resolveCompilationUnit2(source,
+              librarySource);
+          hovers.add(new DartUnitHoverComputer(unit, offset).compute());
+        }
       }
     }
     // send response
     Response response = new Response(request.id);
     response.setResult(HOVERS, hovers);
     return response;
   }
 
   @override
   Response handleRequest(Request request) {
@@ skipping 166 matching lines @@
 
 /**
  * A description of the change to the content of a file.
  */
 class ContentChange {
   String content;
   int offset;
   int oldLength;
   int newLength;
 }