Summary:
Before this commit you could type `Meteor.is` in a `meteor shell` session
and then tab to see a list of possible completions (e.g.
`Meteor.isClient`, `Meteor.isServer`), but typing a prefix of a global
variable name like `Mete` followed by tab has been broken ever since we
stopped using the global object as the REPL context:
https://github.com/meteor/meteor/commit/7c7e52f2d2
The reason for that commit was to prevent the REPL from overwriting the
global `_` variable (which most Meteor developers expect to be bound to
`require("underscore")`): https://github.com/meteor/meteor/3227
This commit solves #3227 by making `repl.context._` a read-only property
that is permanently "bound" to underscore. As a bonus, we now intercept
assignments to `_` and store those values as `repl.context.__`, so you
still have access to the last result in the shell via `__`.
Test Plan:
Run `meteor shell`, evaluate a few expressions, and see that (1) global
variables can be tab-completed, (2) `_` remains bound to underscore, and
(3) `__` gets bound to the result of the evaluated expressions.
Reviewers: avital, stubailo, glasser
Reviewed By: glasser
Differential Revision: https://phabricator.meteor.io/D12
2443d83226