Report HintCode.UNUSED_LOCAL_VARIABLE for local variables whose value is never used.

Report HintCode.UNUSED_LOCAL_VARIABLE for local variables whose value is never used.

R=brianwilkerson@google.com
BUG=

Committed: https://code.google.com/p/dart/source/detail?r=41568

[Brian Wilkerson, 2014-11-06 16:55:01]

LGTM.

As I was looking at this I wondered whether we would want to expand the hint to
find unnecessary assignments, such as on the third line of this code:

var x = 0;
f(x);
x = 1;
return;

If we do, we'll need to take the flow path into account, so keeping that
information in the element won't be sufficient. But that's for later.

[scheglov, 2014-11-06 17:00:35]

Committed patchset #1 (id:1) manually as 41568 (presubmit successful).