repl: pre-define _ in REPL's context

As `_` is not defined in REPL's context, when it is defined as `const`,
it breaks REPL, as it tries to store the result of the last evaluated
expression in `_`. This patch makes sure that `_` is pre-defined in
REPL's context, so that if users define it again, they will get error.

Refer: nodejs#3729
Refer: nodejs#3704

lib/repl.js

@@ -562,6 +562,12 @@ REPLServer.prototype.createContext = function() {
     });
   });
 
+  // Refer: https://github.com/nodejs/node/pull/3729#issuecomment-155460861
+  // The REPL stores the result of the last evaluated expression in context's _.
+  // But, in the REPL, if _ is defined as a const, then it will break the REPL.
+  // So, we define _ first, so that later redefiniitions will fail.
+  context._ = undefined;
+
   return context;
 };