VM should search the SDK for dart: libs that aren't built into the binary

[dgrove]

The VM should support dart:json (and potentially other dart: libraries that aren't built into the binary).

In order to do that, it should be able to search the SDK lib directory for other dart: libraries.

[jmesserly]

fwiw, I think dart:json should be builtin, as it is in V8 & chrome

[iposva-google]

Added Accepted label.

[iposva-google]

The addition of package handling makes this obsolete.

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Added AssumedStale label.