update to runtime v1.6.1, remove submodule in favor of script

The dart binaries are now combined via `libtool` so that applications only need to link to one static library, libdart_runtime.a.  Due to this, the binary size is now above the 100mb cap that github enforces, so they cannot be checked in.  So, I'm moving to the good 'ol bash fetch and install script approach.  This was probably inevitable anyway, since libdart_vm.lib on windows was likely to be greater than 100mb.

Aside from this change, this now running with dart runtime 1.6.1, which seems to be pretty stable.