make mixins behave like closures

lib/less/tree/mixin.js

@@ -52,6 +52,7 @@ tree.mixin.Definition = function (name, params, rules) {
         else                    { return count }
     }, 0);
     this.parent = tree.Ruleset.prototype;
+    this.frames = [];
 };
 tree.mixin.Definition.prototype = {
     toCSS: function () { return "" },
@@ -73,7 +74,7 @@ tree.mixin.Definition.prototype = {
             }
         }
         return new(tree.Ruleset)(null, this.rules).evalRules({
-            frames: [this, frame].concat(env.frames)
+            frames: [this, frame].concat(this.frames, env.frames)
         });
     },
     match: function (args, env) {

lib/less/tree/ruleset.js

@@ -108,6 +108,8 @@ tree.Ruleset.prototype = {
             if (this.rules[i] instanceof tree.mixin.Call) {
                 Array.prototype.splice
                      .apply(this.rules, [i, 1].concat(this.rules[i].eval(env)));
+            } else if (this.rules[i] instanceof tree.mixin.Definition) {
+                this.rules[i].frames = env.frames.slice(0);
             }
         }