Rename kDummyTokenIndex to kNoSourcePos

Rename kDummyTokenIndex to kNoSourcePos

Addresses comment to previous change list.

R=regis@google.com

Committed: https://code.google.com/p/dart/source/detail?r=31969

[hausner, 2014-01-21 22:31:11]

Mostly just renaming. No functionality changes.

[regis, 2014-01-21 23:03:33]

This is not exactly what we talked about. The idea was to add an alias, not to
rename the constant.

I thought that we wanted to prevent the debugger from single stepping on certain
valid source positions. The name kNoSourcePos simply means that we do not have a
source position, not that the debugger should not single step at that position.
A code reader may add a valid source position, thinking that it was an
oversight. The idea was to make it clear that stepping is not allowed for some
source positions.

But if you think that the distinction is not very clear or that it is too
painful to revisit all uses, then LGTM.

[hausner, 2014-01-21 23:45:33]

The comment next to the definition of kDummyTokenIndex said "reflecting an
unknown source position". I thought the name kNoSourcePos expresses that more
clearly.

In the previous change I gave a few AST nodes that contain synthesized code
(i.e. there is no source code for them) the dymmy index, which I now rename to
NoSourcePos. It seems to me that all uses of this constant are for cases where
we have no useful source code position of a class, type, function etc.

I hesitate to give the same token index value two different names. We can then
set a token index to kDummyTokenIndex and interpret it as kNoSourcePos in the
debugger.

[hausner, 2014-01-21 23:46:43]

Committed patchset #2 manually as r31969 (presubmit successful).