Search should not show results in SDK .dartdoc files

[pq]

This is actually a symptom of the larger issue that the SDK ships lots of artifacts that we don't want to traverse in search.

I'd like to approach this in two passes:

1. remove what we can from the SDK
2. filter the rest

[pq]

Actually, the issue transcends .dartdoc and really embraces all non-darty SDK citizens with extensions like .c, .cc, .gypi, .h, .py, etc. The fix is easy (whitelist the good extensions) and on the way.

---

Added Started label.

[pq]

Fixed w/ r9257 (http://codereview.chromium.org/10694039/).

---

Added Fixed label.