Add dart2snapshot compiler and dart_snapshotted runtime to test scripts

tests/lib/lib.status

Index: tests/lib/lib.status
diff --git a/tests/lib/lib.status b/tests/lib/lib.status
index d854ac9c2c0f8c97b9765a0601f12afb4185ea0e..a040614d8a0bda820f09aa6a0864fe69f8a0ef5d 100644
--- a/tests/lib/lib.status
+++ b/tests/lib/lib.status
@@ -188,6 +188,12 @@ async/timer_not_available_test: Fail, OK # only meant to test when there is no w
 [ ($runtime == vm || $runtime == dart_precompiled) ]
 js/*: Skip
 
+# 'js' tests import the dart:js library, so they only make sense in
+# a browser environment.
+[ $runtime == dart_snapshotted ]
+js/*: SkipByDesign
+mirrors/*: SkipByDesign
+
 [ $compiler == dart2js && $minified ]
 mirrors/mirrors_used_get_name_test: RuntimeError
 mirrors/mirrors_used_get_name2_test: RuntimeError