VM: Normalize function result type void to null in the flow graph.

VM: Normalize function result type void to null in the flow graph.

When computing the compile-type there should never be a value of
void type. The only value assignable to void is null, so the compiler
can use null-type as compile-type for such a value.

Before, the compiler would crash with an assertion-failure in checked
mode when running with the flag --no-eliminate-type-checks and assigning
the result of a static void function to a variable of some type.

This CL fixes the issue no matter how the flag is set. We could separately
consider removing this flag (it's default is true).

R=vegorov@google.com

Committed: https://code.google.com/p/dart/source/detail?r=41563

[Florian Schneider, 2014-11-06 13:49:49]

fschneider@google.com changed reviewers:
+ vegorov@google.com

[Vyacheslav Egorov (Google), 2014-11-06 13:52:29]

lgtm

[Florian Schneider, 2014-11-06 15:24:09]

PTAL

[Vyacheslav Egorov (Google), 2014-11-06 15:27:14]

lgtm

[Florian Schneider, 2014-11-06 15:40:53]

Committed patchset #2 (id:20001) manually as r41563 (presubmit successful).