Provide an API to enable Observatory from within the Dart process

[kevmoo]

We want to be able to ship commands, like 'pub run test' or even globally activated tools that can opt-in to having the observatory running while running.

It's also important that the process can know which port observatory is listening on without having to parse stdout at runtime.

A top-level method in dart:io (or even a static on Platform)

Uri enableVMService({int port})

or similar

[kevmoo]

Blocking dart-lang/test#50

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Added this to the 1.11 milestone.
Removed Priority-Unassigned label.
Added Priority-High label.

[iposva-google]

This is currently not a planned feature.

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Removed this from the 1.11 milestone.
Removed Priority-High label.
Added Priority-Unassigned, NotPlanned labels.

[johnmccutchan]

https://codereview.chromium.org/2438613002

[nex3]

Where are the tests? I only see one file with maybe 20 lines of actual code. Where are the tests that requests with invalid auth codes don't go through? What happens if enable is false? What happens if enable is true and there's already a server? Where are the tests for actually accessing members of ServiceProtocolInfo rather than just printing the toString()?

This also seems substantially under-documented. There's no documentation for the enable parameter, there's no indication what hostname or port will be used for the service protocol, there's no way for a new user to figure out what the "service protocol" even is.

[kevmoo]

Tests have been updated by @a-siva b2f3bb6

It'd be great if we could get some improved documentation here.

[itsjoeconway]

Was an option to select port ever added here? Can't find it in API ref.