REPLServer.createContext() has unexpected side effects

[lance]

• Version: 8.1.4
• Platform: Darwin Callisto 16.6.0 Darwin Kernel Version 16.6.0: Fri Apr 14 16:21:16 PDT 2017; root:xnu-3789.60.24~6/RELEASE_X86_64 x86_64
• Subsystem: repl

Calling the undocumented REPLServer.createContext() has unexpected side effects. When the method is called, the value for the server instance's underscoreAssigned and lines properties reset. Here is a reproducible test case.

const repl = require('repl');
const util = require('util');
const assert = require('assert');

const server = repl.start({ prompt: '> ' });
assert.ok(!server.underscoreAssigned);
assert.strictEqual(server.lines.length, 0);

server.write('_ = 500;\n');
assert.ok(server.underscoreAssigned);
assert.strictEqual(server.lines.length, 1);

const context = server.createContext();

assert.ok(server.underscoreAssigned);
assert.strictEqual(server.lines.length, 1);

The overwriting of underscoreAssigned was introduced in ad8257f, while the overwriting of lines is 6 years old at least.

I discovered this when looking for usages of createContext() in the interest of deprecating it and REPLServer.resetContext() for #7619.