created assert stub for the browser

browser/assert.js

@@ -0,0 +1,7 @@
+
+var assert = {
+  ok: function(value, message) {
+    message = message || '';
+    if(!value) throw new Error("assertion failed", message);
+  }
+}

lib/carto/tree/definition.js

@@ -1,7 +1,5 @@
 (function(tree) {
-var assert = require('assert') || {
-    ok: function() { }
-};
+var assert = require('assert');
 
 tree.Definition = function Definition(selector, rules) {
     this.elements = selector.elements;