Moves the 'import_*.config' files in dart-sdk into a new directory

Moves the 'import_*.config' files in dart-sdk into a new directory

These files are used by the static analyzer and editor to target
different implementations of the system library.

This change moves these files from lib/config to lib/_internal/config so 
that there will be no confusion that these might be intended to 
be dart system library files.

http://code.google.com/p/dart/issues/detail?id=2510

Committed: https://code.google.com/p/dart/source/detail?r=6543

[danrubel, 2012-04-13 19:04:02]

lgtm