Don't use instanceof to determine if a Context is a Context

Fixes linkedin#743

lib/dust.js

@@ -328,12 +328,17 @@
     this.options = options;
     this.blocks = blocks;
     this.templateName = templateName;
+    this._isContext = true;
   }
 
   dust.makeBase = dust.context = function(global, options) {
     return new Context(undefined, global, options);
   };
 
+  dust.isContext = function(obj) {
+    return typeof obj === "object" && obj._isContext === true;
+  }
+
   /**
    * Factory function that creates a closure scope around a Thenable-callback.
    * Returns a function that can be passed to a Thenable that will resume a
@@ -347,8 +352,7 @@
   }
 
   Context.wrap = function(context, name) {
-    if (context instanceof Context) {
-      context.templateName = name;
+    if (dust.isContext(context)) {
       return context;
     }
     return new Context(context, {}, {}, null, name);