Set predefined environment variables for existing libraries in the VM

[floitschG]

See issue #24581.

Not clear, if we want to expose these behind the flag, or not.

[lrhn]

It's not clear what happens if the user adds -Ddart.library.io=false on the command line.
Should it override the default, not override the default or cause an error?

[floitschG]

Explicitly provided flags should override the automatically provided ones.

[floitschG]

Fixed with 55630a5