Support 'dart.library.X' env variables in the VM.

tests/language/language_dart2js.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.
 
 [ $compiler == dart2js ]
 getter_setter_in_lib_test: Fail # Issue 23288
 method_name_test: Fail # issue 25574
 
 async_star_cancel_while_paused_test: RuntimeError # Issue 22853
 async_star_await_pauses_test: Fail, Timeout # Issue 22853
 tearoff_basic_test: Skip # Tear-off not supported
 tearoff_constructor_basic_test: Skip # Tear-off not supported
 
 try_catch_on_syntax_test/10: Fail # Issue 19823
 try_catch_on_syntax_test/11: Fail # Issue 19823
 
 deep_nesting1_negative_test: Crash # Issue 25557
 deep_nesting2_negative_test: Crash # Issue 25557
 
 call_function_apply_test: RuntimeError # Issue 23873
 
 # The following tests are supposed to fail.
-# When run for testing dart2js supports all dart:X libraries (because it
+# In testing-mode, dart2js supports all dart:X libraries (because it
 # uses '--categories=all').
 library_env_test/has_no_html_support: RuntimeError, OK
 library_env_test/has_no_io_support: RuntimeError, OK
 library_env_test/has_no_mirror_support: RuntimeError, OK
 
 [ $compiler == dart2js && $runtime == jsshell ]
 await_for_test: Skip # Jsshell does not provide periodic timers, Issue 7728
 async_star_test: RuntimeError # Jsshell does not provide non-zero timers, Issue 7728
 
 [ $compiler == dart2js && $browser ]
@@ skipping 276 matching lines @@
 
 [ $compiler == dart2js && $cps_ir && $host_checked == false ]
 regress_21795_test: Pass, RuntimeError # Due to inlining?
 
 [ $compiler == dart2js && $cps_ir && $host_checked ]
 async_throw_in_catch_test/forceAwait: Crash # Issue 24485
 async_throw_in_catch_test/none: Crash # Issue 24485
 execute_finally9_test: Crash # Issue 24485
 regress_21795_test: Crash # Issue 24485
 regress_23537_test: Crash # Issue 24485