Fix for NPE if subscribing for errors in a not analyzed file.

pkg/analysis_server/test/domain_analysis_test.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 test.domain.analysis;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analysis_server/src/computer/element.dart';
@@ skipping 307 matching lines @@
       expect(highlights, isEmpty);
       // subscribe
       helper.addAnalysisSubscriptionHighlights(helper.testFile);
       // wait, has regions
       return helper.waitForOperationsFinished().then((_) {
         var highlights = helper.getHighlights(helper.testFile);
         expect(highlights, isNot(isEmpty));
       });
     });
   });
+
+  test('after analysis, no such file', () {
+    AnalysisTestHelper helper = new AnalysisTestHelper();
+    helper.createSingleFileProject('int V = 42;');
+    return helper.waitForOperationsFinished().then((_) {
+      String noFile = '/no-such.file.dart';
+      helper.addAnalysisSubscriptionErrors(noFile);
+      return helper.waitForOperationsFinished().then((_) {
+        var errors = helper.getErrors(noFile);
+        expect(errors, isEmpty);
+      });
+    });
+  });
 }
 
 
 int _getSafeInt(Map<String, Object> json, String key, int defaultValue) {
   Object value = json[key];
   if (value is int) {
     return value;
   }
   return defaultValue;
 }
@@ skipping 139 matching lines @@
       files = <String>[];
       analysisSubscriptions[service.name] = files;
     }
     files.add(file);
     // set subscriptions
     Request request = new Request('0', ANALYSIS_SET_SUBSCRIPTIONS);
     request.setParameter(SUBSCRIPTIONS, analysisSubscriptions);
     handleSuccessfulRequest(request);
   }
 
+  void addAnalysisSubscriptionErrors(String file) {
+    addAnalysisSubscription(AnalysisService.ERRORS, file);
+  }
+
   void addAnalysisSubscriptionHighlights(String file) {
     addAnalysisSubscription(AnalysisService.HIGHLIGHTS, file);
   }
 
   void addAnalysisSubscriptionNavigation(String file) {
     addAnalysisSubscription(AnalysisService.NAVIGATION, file);
   }
 
   /**
    * Creates an empty project `/project`.
@@ skipping 129 matching lines @@
     return waitForServerOperationsPerformed(server);
   }
 
   static String _getCodeString(code) {
     if (code is List<String>) {
       code = code.join('\n');
     }
     return code as String;
   }
 }