Update test expectations for new dart2dart backend.

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.
 
 [ $unchecked ]
 mirrors/redirecting_factory_different_type_test: SkipByDesign # Tests type checks.
 
 [ $csp ]
 # Deferred loading does not work from an isolate in CSP-mode
 async/deferred/deferred_in_isolate_test: Skip # Issue 16898
@@ skipping 270 matching lines @@
 mirrors/lazy_static_test: SkipSlow # Times out. Issue 19127
 mirrors/mirrors_reader_test: SkipSlow # Times out. Issue 19127
 
 [ $compiler == dart2js && $runtime == safarimobilesim ]
 mirrors/mirrors_reader_test: SkipSlow # Times out. Issue 20806.
 
 [ $compiler == dart2dart ]
 # Deferred loading is not supported by dart2dart.
 async/deferred/deferred_in_isolate_test: Skip # Issue 17523
 
+[ $compiler == dart2dart && $builder_tag == new_backend ]
+async/future_test: RuntimeError # Issue 23015
+
 [ $compiler == none && ($runtime == drt || $runtime == dartium || $runtime == ContentShellOnAndroid) ]
 # Dart vm does not support spawn from a dom-enabled isolate.
 async/deferred/deferred_in_isolate_test: RuntimeError # Issue 16209
 
 [ $compiler == dart2js && $runtime == jsshell ]
 async/schedule_microtask_test: Fail  # Preamble file does not correctly implement scheduleImmediate.
 
 [ $compiler == dartanalyzer || $compiler == dart2analyzer ]
 mirrors/generic_f_bounded_mixin_application_test: StaticWarning # Test Issue
 
@@ skipping 36 matching lines @@
 
 [ $mode == debug && $arch == ia32 && $system == windows ]
 convert/streamed_conversion_json_utf8_decode_test: Skip  # Verification OOM.
 
 [ $mode == debug && $arch != ia32 && $arch != x64 && $arch != simarm ]
 convert/streamed_conversion_json_utf8_decode_test: Skip  # Verification not yet implemented.
 
 [ $runtime == vm && $mode == debug && $builder_tag == asan ]
 mirrors/immutable_collections_test: SkipSlow  # Timeout.
 convert/streamed_conversion_json_utf8_decode_test: Skip  # Timeout.