Add offline mode

[RossComputerGuy]

• Adds an --offline flag which is useful for sandbox/networkless build systems such as Nix.

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[bkonyi]

Sorry for the delay on this @RossComputerGuy! Overall, this change looks good to me, but we should add some tests before landing.

[RossComputerGuy]

I've gone ahead and rebased it. I will look into getting tests in shortly.

[RossComputerGuy]

Can't figure out how the test should be done, I see there's configuration tests but none invoke read.

[bkonyi]

Can't figure out how the test should be done, I see there's configuration tests but none invoke read.

I think you can either add a test like this or something similar to what's done in this file.

Basically, you'll just want a test that runs webdev serve --no-pub on a project that hasn't had dart pub get invoked to pull down the dependencies. The webdev serve --no-pub should fail to serve the project since there will be unresolved dependencies.

[RossComputerGuy]

Ok, I added the test.