Added async task test

package.json

@@ -19,7 +19,8 @@
 		"gulp-uglify": "~0.3.1",
 		"gulp-minify-html": "~0.1.1",
 		"gulp-minify-css": "~0.3.0",
-		"gulp-rev": "~0.4.2"
+		"gulp-rev": "~0.4.2",
+		"gulp-less": "2.0.1"
 	},
 	"scripts": {
 		"test": "mocha"

test/fixtures/async-less.html

@@ -0,0 +1,4 @@
+<!-- build:less /style.css -->
+<link rel="stylesheet" href="less/clear.less"/>
+<link rel="stylesheet" href="less/main.less"/>
+<!-- endbuild -->

test/fixtures/less/clear.less

@@ -0,0 +1,6 @@
+@zero: 0;
+
+* {
+	margin: @zero;
+	padding: @zero;
+}

test/fixtures/less/main.less

@@ -0,0 +1,3 @@
+body {
+	margin: 10px;
+}

test/main.js

@@ -489,5 +489,25 @@ describe('gulp-usemin', function() {
         );
       });
     });
+
+    it('async task', function(done) {
+      var less = require('gulp-less');
+      var cssmin = require('gulp-minify-css');
+      var stream = usemin({
+        less: [less(), 'concat', cssmin()]
+      });
+
+      var name = 'style.css';
+      var expectedName = 'min-style.css';
+
+      stream.on('data', function(newFile) {
+        if (path.basename(newFile.path) === path.basename(name)) {
+          assert.equal(String(getExpected(expectedName).contents), String(newFile.contents));
+          done();
+        }
+      });
+
+      stream.write(getFixture('async-less.html'));
+    });
   });
 });