Update status for Dartium tests.

tests/lib/lib.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.
 
 [ $checked && $compiler != dartanalyzer && $compiler != dart2analyzer && $runtime != dartium ]
 mirrors/regress_16321_test/01: CompileTimeError # Issue 16351
 
 [ $checked && $compiler == none && ($runtime == dartium || $runtime == drt) ]
 mirrors/regress_16321_test/01: RunTimeError # Issue 16351
 
@@ skipping 237 matching lines @@
 [ $compiler == none && $runtime == drt && $system == windows ]
 async/multiple_timer_test: Fail, Pass # See Issue 10982
 async/timer_test: Fail, Pass # See Issue 10982
 
 [ $compiler == none && $runtime == dartium ]
 async/schedule_microtask5_test: Pass, Timeout # Issue 13719: Please triage this failure.
 async/timer_cancel2_test: Pass, Timeout # Issue 13719: Please triage this failure.
 
 [$compiler == none && $runtime == ContentShellOnAndroid ]
 async/stream_timeout_test: RuntimeError, Pass # Issue 19127
+async/slow_consumer3_test: Skip # Times out flakily. Issue 20956
 convert/streamed_conversion_utf8_encode_test: Skip # Times out or passes. Issue 19127
 convert/streamed_conversion_utf8_decode_test: Skip # Times out or passes. Issue 19127
 mirrors/lazy_static_test: Skip # Times out. Issue 19127
 mirrors/mirrors_reader_test: Skip # Times out. Issue 19127
 
 [ $compiler == dart2js && $runtime == safarimobilesim ]
 mirrors/mirrors_reader_test: Skip # Times out. Issue 20806.
 
 [ $compiler == dart2dart ]
 # Deferred loading is not supported by dart2dart.
@@ skipping 37 matching lines @@
 
 [ $arch == simarm ]
 convert/chunked_conversion_utf88_test: Skip  # Pass, Slow Issue 12644.
 convert/utf85_test: Skip  # Pass, Slow Issue 12644.
 
 [ $compiler == dart2js ]
 profiler/metrics_num_test: Skip # Because of a int / double type test.
 
 [ $arch == simarm64 ]
 convert/utf85_test: Skip # Pass, Slow Issue 20111.