Make sure variables named e.g. "constructor" are declared with var

lib/nodes.js

@@ -895,7 +895,7 @@
     // equivalent syntax tree and compile that, in pieces. You can see the
     // constructor, property assignments, and inheritance getting built out below.
     ClassNode.prototype.compileNode = function(o) {
-      var _b, _c, _d, _e, access, applied, className, constScope, construct, extension, func, me, pname, prop, props, pvar, returns, val;
+      var _b, _c, _d, _e, access, applied, className, constScope, construct, constructor, extension, func, me, pname, prop, props, pvar, returns, val;
       extension = this.parent && new ExtendsNode(this.variable, this.parent);
       props = new Expressions();
       o.top = true;

lib/scope.js

@@ -62,7 +62,7 @@
     };
     // Just check to see if a variable has already been declared, without reserving.
     Scope.prototype.check = function(name) {
-      if (this.variables[name]) {
+      if (this.variables.hasOwnProperty(name)) {
         return true;
       }
       return !!(this.parent && this.parent.check(name));

src/scope.coffee

@@ -46,7 +46,7 @@ exports.Scope: class Scope
 
   # Just check to see if a variable has already been declared, without reserving.
   check: (name) ->
-    return true if @variables[name]
+    return true if @variables.hasOwnProperty name
     !!(@parent and @parent.check(name))
 
   # If we need to store an intermediate result, find an available name for a