Mark pkg/csslib/test/declaration_test and pkg/csslib/test/mixin_test as failing on ie9 and ie10

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 187 matching lines @@
 
 [ $compiler == dart2js || $compiler == dart2dart ]
 source_maps/test/vlq_test: RuntimeError # A VLQ test checks for large numbers that
                                 # overflow in JS (numbers slightly larger than
                                 # 32 bits where we do bitwise operations).
 
 [ $runtime == opera && $compiler == dart2js ]
 intl/test/find_default_locale_browser_test: Fail
 intl/test/date_time_format_http_request_test: Skip # Timeout.
 
+[ $runtime == ie9 || $runtime == ie10 ]
+csslib/test/declaration_test: RuntimeError # Issue 20274
+csslib/test/mixin_test: RuntimeError # Issue 20274
+
 [ $runtime == ie9 ]
 mime/test/mime_multipart_transformer_test: Skip # No typed_data on IE9.
 typed_data/test/typed_buffers_test: Fail, Crash # No typed_data on IE9.
 polymer/test/instance_attrs_test: Pass, Fail # Issue 14167
 polymer/test/bind_mdv_test: Slow, Pass, RuntimeError # Issue 14412, 13260
 matcher/test/core_matchers_test: RuntimeError # Issue 17762
 polymer_expressions/test/bindings_test: RuntimeError # Issue 19265
 polymer_expressions/test/globals_test: RuntimeError # Issue 19265
 template_binding/test/template_binding_test: Pass, RuntimeError # Issue 19265
 
@@ skipping 249 matching lines @@
 oauth2/test/handle_access_token_response_test: Fail
 
 [ $runtime == drt ]
 http/test/html/client_test: Skip # Issue 18566
 
 [ $system == windows ]
 analysis_server/test/integration: Skip # Issue 20252
 
 [ $mode == debug ]
 analysis_server/test/integration/analysis_domain_int_test: Slow, Pass