Apply Mockito-related type fixes (#16002)

Type-related fixes in flutter_driver and fuchsia_remote_debug_protocol
tests.

packages/flutter_driver/test/flutter_driver_test.dart

@@ -37,9 +37,9 @@ void main() {
       mockVM = new MockVM();
       mockIsolate = new MockIsolate();
       mockPeer = new MockPeer();
-      when(mockClient.getVM()).thenReturn(mockVM);
+      when(mockClient.getVM()).thenReturn(new Future<MockVM>.value(mockVM));
       when(mockVM.isolates).thenReturn(<VMRunnableIsolate>[mockIsolate]);
-      when(mockIsolate.loadRunnable()).thenReturn(mockIsolate);
+      when(mockIsolate.loadRunnable()).thenReturn(new Future<MockIsolate>.value(mockIsolate));
       when(mockIsolate.invokeExtension(typed(any), typed(any))).thenAnswer(
           (Invocation invocation) => makeMockResponse(<String, dynamic>{'status': 'ok'}));
       vmServiceConnectFunction = (String url) {

packages/fuchsia_remote_debug_protocol/test/fuchsia_remote_connection_test.dart

@@ -31,7 +31,8 @@ void main() {
       const String interface = 'eno1';
       // Adds some extra junk to make sure the strings will be cleaned up.
       when(mockRunner.run(typed(any)))
-          .thenReturn(<String>['123\n\n\n', '456  ', '789']);
+      .thenReturn(new Future<List<String>>.value(
+          <String>['123\n\n\n', '456  ', '789']));
       when(mockRunner.address).thenReturn(address);
       when(mockRunner.interface).thenReturn(interface);
       int port = 0;

packages/fuchsia_remote_debug_protocol/test/src/runners/ssh_command_runner_test.dart

@@ -40,7 +40,8 @@ void main() {
     setUp(() {
       mockProcessManager = new MockProcessManager();
       mockProcessResult = new MockProcessResult();
-      when(mockProcessManager.run(typed(any))).thenReturn(mockProcessResult);
+      when(mockProcessManager.run(typed(any))).thenReturn(
+          new Future<MockProcessResult>.value(mockProcessResult));
     });
 
     test('verify interface is appended to ipv6 address', () async {