Eliminate unary plus with hex literals

Eliminate unary plus with hex literals

The spec says unary + is only allowed with integer and floating point literals, not with hex literals. Somebody noticed that the VM accepted
+0x10 as well.

Consequently, the Math.parseInt(str) library function should fail with
a BadNumberFormatException if it is called with a hex number that is preceded by a +.

Fixes issue 1540.

This change exposed that the dart2js core library implementation is
falsely accepting +0xNNN hex numbers. I filed issue 3333.
Committed: https://code.google.com/p/dart/source/detail?r=8177

[hausner, 2012-05-31 18:30:51]

Itsy bitsy review.

[srdjan, 2012-05-31 19:06:22]

LGTM after testing there are no failures in other components.