Fix Windows path handling in uri_resolver_test.

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 33 matching lines @@
 
 [ $runtime == vm && $checked ]
 analysis_server/test/completion_test: Pass, Slow
 
 [ $runtime == vm && $system == windows]
 analysis_server/*: Skip # Issue 27557
 analysis_server/test/analysis/get_errors_test: Skip # runtime error, Issue 22180
 analysis_server/test/integration/analysis/analysis_options_test: RuntimeError # Issue 24796
 analyzer/test/generated/all_the_rest_test: Fail # Issue 21772
 analyzer/test/generated/source_factory_test: RuntimeError # Issue 26828
-front_end/test/src/base/uri_resolver_test: RuntimeError # Issue 28309
 kernel/test/baseline_type_propagation_test: RuntimeError # Issue 28243
 kernel/test/baseline_spec_mode_test: RuntimeError # Issue 28243
 kernel/test/baseline_strong_mode_test: RuntimeError # Issue 28243
 
 [ $compiler == dart2js ]
 analysis_server/test/integration: SkipByDesign # Analysis server integration tests don't make sense to run under dart2js, since the code under test always runs in the Dart vm as a subprocess.
 analyzer_cli/test/*: SkipByDesign # Only meant to run on vm
 collection/test/equality_test/01: Fail # Issue 1533
 collection/test/equality_test/02: Fail # Issue 1533
 collection/test/equality_test/03: Fail # Issue 1533
@@ skipping 147 matching lines @@
 analyzer/test/src/task/yaml_test: Crash # Issue 24485
 
 [ $noopt || $runtime == dart_precompiled ]
 *: SkipByDesign # The pkg test framework imports dart:mirrors.
 
 [ $compiler == dart2js && $cps_ir && $checked ]
 *: Skip # `assert` not implemented, about 75% tests crash
 
 [ $compiler == none ]
 kernel/test/closures_test: Fail