Add type check to const conditional expression evaluation

Add type check to const conditional expression evaluation

issue 13746

R=srdjan@google.com

Committed: https://code.google.com/p/dart/source/detail?r=28182

[srdjan, 2013-10-02 17:55:12]

Isn't the compilation error confusing now "expression must be a compile-time
constant")? It should be reporting that a boolean compile-time constant is
expected.

[hausner, 2013-10-02 18:14:01]

We don't have the ability to report why an expression is not a compile-time
constant at this point.

[hausner, 2013-10-02 18:19:49]

Changed the error message to "is not a valid compile-time constant".

[srdjan, 2013-10-02 18:51:07]

lgtm

[hausner, 2013-10-02 21:08:29]

Committed patchset #3 manually as r28182 (presubmit successful).