Fix #3804: Provide list of referenced vars to REPL

Supersedes #3805. Here is a comparison of master, #3805 and this commit:

    # master
    $ bin/coffee
    coffee> 1 %% 2
    TypeError: Array.prototype.indexOf called on null or undefined

    # #3805
    $ bin/coffee
    coffee> 1 %% 2
    1
    coffee> (_results = null; i) for i in [1, 2, 3]
    TypeError: Cannot call method 'push' of null

    # this commit
    $ bin/coffee
    coffee> 1 %% 2
    1
    coffee> (_results = null; i) for i in [1, 2, 3]
    [ 1, 2, 3 ]

lib/coffee-script/repl.js

@@ -19,16 +19,29 @@
     historyFile: process.env.HOME ? path.join(process.env.HOME, '.coffee_history') : void 0,
     historyMaxInputSize: 10240,
     "eval": function(input, context, filename, cb) {
-      var Assign, Block, Literal, Value, ast, err, js, result, _ref1;
+      var Assign, Block, Literal, Value, ast, err, js, referencedVars, result, token, tokens, _ref1;
       input = input.replace(/\uFF00/g, '\n');
       input = input.replace(/^\(([\s\S]*)\n\)$/m, '$1');
       _ref1 = require('./nodes'), Block = _ref1.Block, Assign = _ref1.Assign, Value = _ref1.Value, Literal = _ref1.Literal;
       try {
-        ast = CoffeeScript.nodes(input);
+        tokens = CoffeeScript.tokens(input);
+        referencedVars = (function() {
+          var _i, _len, _results;
+          _results = [];
+          for (_i = 0, _len = tokens.length; _i < _len; _i++) {
+            token = tokens[_i];
+            if (token.variable && token[1].charAt(0) === '_') {
+              _results.push(token[1]);
+            }
+          }
+          return _results;
+        })();
+        ast = CoffeeScript.nodes(tokens);
         ast = new Block([new Assign(new Value(new Literal('_')), ast, '=')]);
         js = ast.compile({
           bare: true,
-          locals: Object.keys(context)
+          locals: Object.keys(context),
+          referencedVars: referencedVars
         });
         result = context === global ? vm.runInThisContext(js, filename) : vm.runInContext(js, context, filename);
         return cb(null, result);

src/repl.coffee

@@ -20,13 +20,19 @@ replDefaults =
     {Block, Assign, Value, Literal} = require './nodes'
 
     try
-      # Generate the AST of the clean input.
-      ast = CoffeeScript.nodes input
+      # Tokenize the clean input.
+      tokens = CoffeeScript.tokens input
+      # Collect referenced variable names just like in `CoffeeScript.compile`.
+      referencedVars = (
+        token[1] for token in tokens when token.variable and token[1].charAt(0) is '_'
+      )
+      # Generate the AST of the tokens.
+      ast = CoffeeScript.nodes tokens
       # Add assignment to `_` variable to force the input to be an expression.
       ast = new Block [
         new Assign (new Value new Literal '_'), ast, '='
       ]
-      js = ast.compile bare: yes, locals: Object.keys(context)
+      js = ast.compile {bare: yes, locals: Object.keys(context), referencedVars}
       result = if context is global
         vm.runInThisContext js, filename
       else