Remove support for operator negate and replace occurrences with unary

Remove support for operator negate and replace occurrences with unary 
operator -.

In the VM, I have disabled the operator negate support in the parser but I 
have not removed the keyword because the token is used in quite a few other
places.

R=ahe@google.com,iposva@google.com,hausner@google.com
BUG=

Committed: https://code.google.com/p/dart/source/detail?r=12233

[kasperl, 2012-09-11 10:48:10]

Adding Brian as reviewer for the compiler/ parts.

[kasperl, 2012-09-11 10:48:42]

Woops. Meant: Adding Brian as reviewer for the editor parts.

[ahe, 2012-09-11 13:13:30]

LGTM

[hausner, 2012-09-11 16:59:20]

VM changes LGTM. I can remove the remaining occurrences of kNEGATE.