This CL factors the network interactions out of DartLoader into a
DartLibraryProvider interface, paving the way for other library providers
(e.g., offline).
As part of this CL, I've renamed DartLoader to DartLibraryLoader and moved the
class into tonic, where it can't have any direct network dependencies.
R=eseidel@chromium.org
Review URL: https://codereview.chromium.org/1202283004.
fa627b4b0e