Improve dlsym() error-detection.

Improve dlsym() error-detection.

Avoid false-positive error detection by calling dlerror() once (and
ignoring the result) before calling dlsym().

See "man dlsym" on a Linux machine for more discussion of how to detect
errors when calling dlsym().

Committed: https://code.google.com/p/dart/source/detail?r=12504

[jackpal, 2012-09-17 23:52:17]

Assuming Bill Hesse would be a good reviewer for this as he wrote the
documentation on Dart VM Native Extensions.

Please re-assign as appropriate.

[William Hesse, 2012-09-18 08:32:49]

LGTM.  Thanks.