fixed a bug when using @media with mixins

lib/less/tree/media.js

@@ -22,22 +22,23 @@ tree.Media.prototype = {
             env.mediaBlocks = [];
             env.mediaPath = [];
         }
-
-        env.mediaBlocks.push(this);
+        
+        var blockIndex = env.mediaBlocks.length;
         env.mediaPath.push(this);
+        env.mediaBlocks.push(this);
 
-        this.features = this.features.eval(env);
-        env.frames.unshift(this);
-        this.ruleset = this.ruleset.eval(env);
+        var media = new(tree.Media)([], []);
+        media.features = this.features.eval(env);
+        
+        env.frames.unshift(this.ruleset);
+        media.ruleset = this.ruleset.eval(env);
         env.frames.shift();
-
+        
+        env.mediaBlocks[blockIndex] = media;
         env.mediaPath.pop();
 
-        if (env.mediaPath.length === 0) {
-            return this.evalTop(env);
-        } else {
-            return this.evalNested(env);
-        }
+        return env.mediaPath.length === 0 ? media.evalTop(env) :
+                    media.evalNested(env)
     },
     variable: function (name) { return tree.Ruleset.prototype.variable.call(this.ruleset, name) },
     find: function () { return tree.Ruleset.prototype.find.apply(this.ruleset, arguments) },
@@ -93,9 +94,9 @@ tree.Media.prototype = {
         return new(tree.Ruleset)([], []);
     },
     permute: function (arr) {
-      if (arr.length == 0) {
+      if (arr.length === 0) {
           return [];
-      } else if (arr.length == 1) {
+      } else if (arr.length === 1) {
           return arr[0];
       } else {
           var result = [];

test/css/media.css

@@ -48,3 +48,29 @@
     width: 100%;
   }
 }
+.a {
+  background: black;
+}
+@media handheld {
+  .a {
+    background: white;
+  }
+}
+@media handheld and (max-width: 100px) {
+  .a {
+    background: red;
+  }
+}
+.b {
+  background: black;
+}
+@media handheld {
+  .b {
+    background: white;
+  }
+}
+@media handheld and (max-width: 200px) {
+  .b {
+    background: red;
+  }
+}

test/less/media.less

@@ -52,4 +52,24 @@ body {
             width: 100%;
         }
     }
+}
+
+.mediaMixin(@fallback: 200px) {
+    background: black;
+
+    @media handheld {
+        background: white;
+
+        @media (max-width: @fallback) {
+            background: red;
+        }
+    }
+}
+
+.a {
+  .mediaMixin(100px);
+}
+
+.b {
+  .mediaMixin();
 }