Delete Sky tests that we're not going to run with Dart

sky/tests/mutation-observer/observe-attributes-expected.txt

-ERROR: Failed to load resource: the server responded with a status of 404 () 
-SOURCE: http://127.0.0.1:8000/sky/tests/mutation-observer/baz.png:0
-Running 14 tests
-ok 1 MutationObserver.observe on attributes should handle basic aspects of attribute observation
-ok 2 MutationObserver.observe on attributes should not notify of attribute changes without asking
-ok 3 MutationObserver.observe on attributes re-observing the same node with the same observer has the effect of resetting the options
-ok 4 MutationObserver.observe on attributes multiple observers can be registered to a given node and both receive mutations
-ok 5 MutationObserver.observe on attributes should deliver mutations on modifications to node properties which delegate to attribute storage
-ok 6 MutationObserver.observe on attributes should handle basic oldValue delivery
-ok 7 MutationObserver.observe on attributes should deliver oldValue when needed
-ok 8 MutationObserver.observe on attributes should give attributeOldValue if any entries request it with multiple observers
-ok 9 MutationObserver.observe on attributes should handle setting an attribute via reflected IDL attribute
-ok 10 MutationObserver.observe on attributes should respect attributeFilter on HTML elements
-ok 11 MutationObserver.observe on attributes should respect different attributeFilters when observing multiple subtree nodes
-ok 12 MutationObserver.observe on attributes should create records for the style property
-ok 13 MutationObserver.observe on attributes should have oldValue for style property mutations
-ok 14 MutationObserver.observe on attributes should not create records for noop style property mutation
-14 tests
-14 pass
-0 fail