Improvements to causal async stack traces

tests/language/language.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.
 
 # This directory contains tests that are intended to show the
 # current state of the language.
 
 # These test entries will be valid for vm/dartium (with and without kernel).
 [ $compiler == none || $compiler == app_jit || $compiler == dartk || $runtime == dart_precompiled ]
 async_star_cancel_while_paused_test: RuntimeError
@@ skipping 170 matching lines @@
 null_test/none: SkipByDesign
 overridden_no_such_method_test: SkipByDesign
 redirecting_factory_reflection_test: SkipByDesign
 regress_13462_0_test: SkipByDesign
 regress_13462_1_test: SkipByDesign
 regress_18535_test: SkipByDesign
 super_call4_test: SkipByDesign
 super_getter_setter_test: SkipByDesign
 vm/reflect_core_vm_test: SkipByDesign
 regress_28255_test: SkipByDesign
+# Causal async stacks are not supported in product mode
 vm/causal_async_exception_stack_test: SkipByDesign
+vm/causal_async_exception_stack2_test: SkipByDesign
 
 [ $mode == product || $compiler == app_jit || $compiler == precompiler ]
 # Deferred loading happens eagerly. Issue #27587
 deferred_load_constants_test/02: Fail
 deferred_load_constants_test/03: Fail
 deferred_load_constants_test/05: Fail
 deferred_not_loaded_check_test: Fail
 vm/regress_27201_test: Fail
 
 [ $compiler == app_jit ]
@@ skipping 114 matching lines @@
 # Use package:unittest
 async_await_test: RuntimeError
 async_star_test: RuntimeError
 # TODO(zra): Investigate
 vm/regress_28325_test: RuntimeError
 closure_cycles_test: Crash
 
 [$compiler == dart2analyzer]
 vm/regress_27201_test: SkipByDesign # Loads bad library, so will always crash.
 config_import_corelib_test: StaticWarning, OK