Improve error handling in the process library.

Improve error handling in the process library.

Verify that the arguments have the types expected by the
implementation. Require a List instance containing builtin
Strings as the arguments. Require a builtin String path.

R=sgjesse@google.com
BUG=
TEST=

Committed: https://code.google.com/p/dart/source/detail?r=690

[Mads Ager (google), 2011-10-25 11:18:42]

This change looks much bigger than it is. It changes all assert occurrences to
ASSERT for consistency.

The real changes are in process.cc, process*.dart and the new test file.

[Søren Gjesse, 2011-10-25 11:32:51]

lgtm

I guess we need to something like this in other places where we pass dart
objects to C++ code.

[Mads Ager (google), 2011-10-25 11:33:28]

Yes, we do need to do that in other places. I'm on it. :-)