Simplify SnapshotReader::LookupInternalClass.

All callers of this function assert that the returned RawClass
is non-null. Also, the returned Class is only used to extract its class id.

I removed the unreachable code and simplified it accordingly to just return
the class id directly.

R=asiva@google.com

Review URL: https://codereview.chromium.org//672183002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41277 260f80e4-7a28-3924-810f-c04153c831b5