Bump mockito to 3.0.0-alpha+3 (#16306)

* Bump mockito to 3.0.0-alpha+3

* Type annotation

packages/flutter_tools/test/version_test.dart

diff --git a/packages/flutter_tools/test/version_test.dart b/packages/flutter_tools/test/version_test.dart
index 8495328..e3f1ed8 100644
--- a/packages/flutter_tools/test/version_test.dart
+++ b/packages/flutter_tools/test/version_test.dart
@@ -396,7 +396,7 @@
     throw new StateError('Unexpected call to Cache.setStampFor(${invocation.positionalArguments}, ${invocation.namedArguments})');
   });
 
-  final Answering syncAnswer = (Invocation invocation) {
+  final Answering<ProcessResult> syncAnswer = (Invocation invocation) {
     bool argsAre(String a1, [String a2, String a3, String a4, String a5, String a6, String a7, String a8]) {
       const ListEquality<String> equality = const ListEquality<String>();
       final List<String> args = invocation.positionalArguments.single;