Added source argument when throwing a ArgParserException.

[fa0311]

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[jakemac53]

I don't think I love the user facing result of this PR, which is that the default exception message ends up with an (at offset <index>) in the description. Lets just go with the original :).

[mosuem]

Closing as the dart-lang/args repository is merged into the dart-lang/core monorepo. Please re-open this PR there!