allow user to set location of corelib files in DartEditor

[DartBot]

This issue was originally filed by mattsh@google.com

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DartEditor currently seems to have baked into itself a particular version of the dart core libraries.

It would be good if the user could set the location of corelib.dart, and have the DartEditor use that, in case they want to use a more recent or private version of corelib. (Especially useful for people working on developing corelib.)

[keertip]

Removed Type-Defect label.
Added Type-Enhancement label.

[danrubel]

I'll handle the non-UI portion of this one.

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Set owner to @danrubel.
Added Accepted label.

[DartBot]

This comment was originally written by mattsh@google.com

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Just fyi, if there was a dart editor config file for this, that would be fine too, it's not necessary to have UI right now I think. (Also, there might be other various config settings we think of that we want to add before there's time to change the UI.)

[whesse]

We also want to be able to add additional core libraries, in the case of the runtime shell dart_vm, which has many OS libraries for file and socket access, and process control, added to the runtime.

[bwilkerson]

We also want to be able to add additional core libraries, in the case of the runtime shell dart_vm, which
has many OS libraries for file and socket access, and process control, added to the runtime.

Are there dart: URI's for these libraries? If not, could there be? It would certainly be a lot cleaner from our perspective to have libraries that are platform-specific be explicitly imported by users. Otherwise we have to provide an extra-lingual way for the user to tell us which set of implicit libraries to compile their code against, and we'd prefer not to have to do that.

[danrubel]

The dart core libraries are now in a "libraries" folder (build 1910) rather than embedded in plugins.
You can modify, replace, and add new "dart:<name>" libraries.

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Added Fixed label.