Reenable tab-completion of global variable names in meteor shell.

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

tools/server/shell.js

@@ -56,7 +56,7 @@ function onConnection(socket) {
       prompt: "> ",
       terminal: true,
       useColors: true,
-      useGlobal: false,
+      useGlobal: true,
       ignoreUndefined: true,
     });
 
@@ -77,6 +77,19 @@ function startREPL(options) {
   // History persists across shell sessions!
   initializeHistory(repl);
 
+  Object.defineProperty(repl.context, "_", {
+    // Force the global _ variable to remain bound to underscore.
+    get: function () { return _; },
+
+    // Expose the last REPL result as __ instead of _.
+    set: function(lastResult) {
+      repl.context.__ = lastResult;
+    },
+
+    enumerable: true,
+    configurable: false
+  });
+
   // Use the same `require` function and `module` object visible to the
   // shell.js module.
   repl.context.require = require;