Revert "Change path-observer to lookup properties aggressively and report errors"

Seems that I broke template_binding tests. The fix should be simple, but I'll revert for now until I got it all working again.

Review URL: https://codereview.chromium.org//210823005

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/observe@34402 260f80e4-7a28-3924-810f-c04153c831b5