Fix test 'platform_executable_test'

Fix test 'platform_executable_test'

If the PATH environmant variable is not passed when overriding the
exnironment for a spawned process on Linux a default is used.

From the man page for 'execvp':

The file is sought in the colon-separated list of directory
pathnames specified in the PATH environment variable.  If this
variable isn't defined, the path list defaults to the current
directory followed by the list of directories returned by
confstr(_CS_PATH).  (This confstr(3) call typically returns the
value "/bin:/usr/bin".)

If one have Dart installed in e.g. /usr/bin the test would fail, as
starting 'dart' would succeed.

R=kevmoo@google.com
BUG=

Committed: https://github.com/dart-lang/sdk/commit/99e9842ccc8b129306852c17ed970773446c818b

[kevmoo, 2015-05-26 19:02:01]

LGTM – I was smoking something

[Søren Gjesse, 2015-05-27 07:58:54]

Committed patchset #1 (id:1) manually as
99e9842ccc8b129306852c17ed970773446c818b (presubmit successful).