Removed handle creation and inlined two field index checks

Removed handle creation and inlined Instance::IsValidNativeIndex().
The Dromaeo dom core benchmark shows significant time spent in it, 
where ~half the time is spent creating a handle.
Committed: https://code.google.com/p/dart/source/detail?r=11673

[Tom Ball, 2012-08-30 23:43:10]

Removed handle creation and inlined Instance::IsValidNativeIndex() and
Instance::IsValidFieldOffset().  The Dromaeo dom core benchmark shows
significant time spent in Instance::IsValidNativeIndex, where ~half the
time is spent creating a handle.

[Tom Ball, 2012-08-30 23:44:13]

Removed handle creation and inlined Instance::IsValidNativeIndex() and
Instance::IsValidFieldOffset().  The Dromaeo dom core benchmark shows
significant time spent in Instance::IsValidNativeIndex, where ~half the
time is spent creating a handle.

[Ivan Posva, 2012-08-31 01:01:46]

LGTM -ip