Create new library behavior is not discoverable in editor

[lukechurch]

What steps will reproduce the problem?

1. File New Application
2. Select 'dart:core' on LHS
3. File New File

Creates a new library with the name of the file. There is no indication that this is what is going to happen. There is no other way of creating a new library from the file menu.

[pq]

Removed Priority-Medium label.
Added Priority-Low label.

[danrubel]

Now creates the file as a new library, which appears in the Libraries view.

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Added Fixed label.