Change path-observer to lookup properties aggressively and report errors
asynchronously. Add patch in smoke to work around a mirrors bug revealed by
these changes.

BUG=
R=jmesserly@google.com

Review URL: https://codereview.chromium.org//211763002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/observe@34399 260f80e4-7a28-3924-810f-c04153c831b5