Test which ensures that the global _ isn't compromised.

As demonstrated in https://github.com/meteor/meteor/issues/9276.

This test wouldn't have caught the regression in the previous solution
since the lack of a TTY in the `self-test` test harness caused the tests
themselves to take the path through `shell-server`'s `evaluateAndExit`
logic, which didn't use the `global` scope in the same way as the
interactive shell.  That is no longer the case as of e0682c553d.

tools/tests/apps/shell/server/main.js

@@ -3,3 +3,14 @@ import { Meteor } from 'meteor/meteor';
 Meteor.startup(() => {
   Meteor.checkMeFromShell = "oky dok";
 });
+
+// Create a global underscore variable which should be preserved,
+// not overriden by the special REPL `_` variable, when a command
+// is executed on the shell.  The method will allow the test to call
+// back and confirm it's still set.
+_ = {_specialUnderscoreTestObject: true };
+Meteor.methods({
+  "__meteor__/__self_test__/shell-tests/underscore"() {
+    return typeof _ === "object" && Object.keys(_);
+  }
+})

tools/tests/shell-tests.js

@@ -18,6 +18,19 @@ selftest.define("meteor shell", function () {
   shell.match('{"server":true}');
   shell.expectExit(0);
 
+  shell = s.run("shell");
+  // Make sure that the special Node REPL _ variable is not stomping on any
+  // global `_` (i.e. Underscore) since the default `repl` behavior sets the
+  // special variable `_` to the result of the last operation. This method
+  // call to the server will make sure our special `_` remains intact after
+  // the shell is launched.
+  shell.write(
+    "Meteor.call('__meteor__/__self_test__/shell-tests/underscore')\n");
+  shell.proc.stdin.end();
+  shell.waitSecs(10);
+  shell.match('["_specialUnderscoreTestObject"]');
+  shell.expectExit(0);
+
   shell = s.run("shell");
   // Now try with a bunch of newlines in the input.
   shell.write("500+\n4000\n+60\n\n+\n7\n");