pkg/unittest: add @proxy to Mock

pkg/pkg.status

 # Copyright (c) 2012, 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.
 
 # Don't run any test-like files that show up in packages directories. It
 # shouldn't be necessary to run "pub install" in these packages, but if you do
 # it shouldn't break the tests.
 */packages/*/*: Skip
 */*/packages/*/*: Skip
 */*/*/packages/*/*: Skip
@@ skipping 285 matching lines @@
 
 [ $compiler == dartanalyzer || $compiler == dart2analyzer ]
 docgen/test/single_library_test: StaticWarning
 http/test/client_test: StaticWarning
 http/test/http_test: StaticWarning
 http/test/mock_client_test: StaticWarning
 http/test/multipart_test: StaticWarning
 http/test/request_test: StaticWarning
 http/test/streamed_request_test: StaticWarning
 unittest/test/matchers_test: StaticWarning, OK # testing error creating abstract class
-unittest/test/mock_test: StaticWarning, OK # testing unimplemented members
 
 [ $runtime == vm && ($system == windows || $system == macos) ]
 watcher/test/*/linux_test: Skip
 
 [ $runtime == vm && ($system == windows || $system == linux) ]
 watcher/test/*/mac_os_test: Skip
 
 [ $runtime == vm && $system == macos ]
 watcher/test/directory_watcher/mac_os_test: Pass, Fail # Issue 15024
 
@@ skipping 26 matching lines @@
 collection/test/algorithms_test: PubGetError
 collection/test/iterable_zip_test: PubGetError
 collection/test/priority_queue_test: PubGetError
 collection/test/unmodifiable_collection_test: PubGetError
 collection/test/wrapper_test: PubGetError
 stack_trace/test/chain_test: PubGetError
 stack_trace/test/frame_test: PubGetError
 stack_trace/test/trace_test: PubGetError
 stack_trace/test/vm_test: PubGetError