Add support for 'dart.library.X' environment variables in dart2js.

tests/language/language_dart2js.status

Index: tests/language/language_dart2js.status
diff --git a/tests/language/language_dart2js.status b/tests/language/language_dart2js.status
index 464943bd418077e092c7b7001bc36af8617ed5ab..9c42fe2e60bf03f8dc108526a78fdb3c4f8bae3a 100644
--- a/tests/language/language_dart2js.status
+++ b/tests/language/language_dart2js.status
@@ -18,6 +18,13 @@ try_catch_on_syntax_test/11: Fail # Issue 19823
 
 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
+# 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