Expose watch handler as a separate method

lib/asset_packager.js

@@ -85,6 +85,17 @@ _extend(AssetPackager.prototype, {
   minifier:  null
 });
 
+AssetPackager.prototype.watchPath = function(assetPath) {
+  var self = this;
+  if (self.watch && !self._watching[assetPath]) {
+    self._watching[assetPath] = 
+      FS.watch(assetPath, { persistent: true }, function() {
+        self.info('changed', assetPath);
+        self.invalidate();
+      });
+  }
+};
+
 /**
  * Returns the asset descriptor for a given source asset. `done()` will be 
  * called with the asset instance. The asset should contain the compiled body
@@ -139,14 +150,8 @@ AssetPackager.prototype.getSourceAsset = function(assetPath, done) {
           analyzer(asset, self, next);
         }
       ], function(err) {
-        if (!err && self.watch && !self._watching[assetPath]) {
-          //console.log('watch', assetPath);
-          self._watching[assetPath] = 
-            FS.watch(assetPath, { persistent: true }, function() {
-              self.info('changed', assetPath);
-              self.invalidate();
-            });
-        }
+        if (err) return done(err);
+        if (!err) self.watchPath(assetPath);
         return done(err, asset);
       });
     });