more progress with the lexer, perhaps it's done

lib/coffee_script/coffee-script.js

@@ -23,7 +23,7 @@
   };
   // Compile a list of CoffeeScript files on disk.
   exports.compile_files = function compile_files(paths, callback) {
-    var coffee, js;
+    var coffee, exit_ran, js;
     js = '';
     coffee = process.createChildProcess(compiler, ['--print'].concat(paths));
     coffee.addListener('output', function(results) {
@@ -31,7 +31,13 @@
         return js += results;
       }
     });
+    // NB: we have to add a mutex to make sure it doesn't get called twice.
+    exit_ran = false;
     return coffee.addListener('exit', function() {
+      if (exit_ran) {
+        return null;
+      }
+      exit_ran = true;
       return callback(js);
     });
   };