Update mockito to be compatible with latest analyzer void changes

Change-Id: I94d8a4eae413df7a5daf5e5da0e42d1de74f43e3
Reviewed-on: https://dart-review.googlesource.com/39847
Commit-Queue: Mike Fairhurst <mfairhurst@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

DEPS

@@ -104,7 +104,7 @@ vars = {
   "markdown_tag": "@1.0.0",
   "matcher_tag": "@0.12.1+4",
   "mime_tag": "@0.9.6",
-  "mockito_tag": "@2.0.2",
+  "mockito_tag": "@a92db054fba18bc2d605be7670aee74b7cadc00a",
   "mustache4dart_tag" : "@v2.1.0",
   "oauth2_tag": "@1.1.0",
   "observatory_pub_packages_rev": "@4c282bb240b68f407c8c7779a65c68eeb0139dc6",

pkg/analysis_server_client/test/analysis_server_client_test.dart

@@ -23,7 +23,7 @@ void main() {
   });
 
   test('test_listenToOutput_good', () async {
-    when(_process.stdout).thenReturn(_goodMessage());
+    when(_process.stdout).thenAnswer((_) => _goodMessage());
 
     final future = serverWrapper.send('blahMethod', null);
     serverWrapper.listenToOutput();
@@ -35,7 +35,7 @@ void main() {
   });
 
   test('test_listenToOutput_error', () async {
-    when(_process.stdout).thenReturn(_badMessage());
+    when(_process.stdout).thenAnswer((_) => _badMessage());
     final future = serverWrapper.send('blahMethod', null);
     future.catchError((e) {
       expect(e, new isInstanceOf<ServerErrorMessage>());
@@ -48,7 +48,7 @@ void main() {
   });
 
   test('test_listenToOutput_event', () async {
-    when(_process.stdout).thenReturn(_eventMessage());
+    when(_process.stdout).thenAnswer((_) => _eventMessage());
 
     void eventHandler(String event, Map<String, Object> params) {
       expect(event, 'fooEvent');