diff --git a/.travis.yml b/.travis.yml
index 56aa74a1..740aeb33 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,7 +1,7 @@
 sudo: false
 language: node_js
 node_js:
-  - 4
+  - node
 addons:
   artifacts:
     paths:
diff --git a/package.json b/package.json
index 04a1dbd1..77ab4c83 100644
--- a/package.json
+++ b/package.json
@@ -14,19 +14,19 @@
     "karma-firefox-launcher": "~1.0.0",
     "karma-mocha": "^1.2.0",
     "karma-phantomjs-launcher": "^1.0.2",
-    "karma-rollup-plugin": "^0.2.4",
+    "karma-rollup-preprocessor": "^4.0.1",
     "karma-safari-launcher": "~1.0.0",
     "leafdoc": "^1.4.1",
     "mocha": "^3.1.0",
     "phantomjs-prebuilt": "^2.1.12",
     "prosthetic-hand": "^1.3.1",
-    "rollup": "^0.41.4",
+    "rollup": "^0.45.2",
     "rollup-plugin-git-version": "0.2.1",
     "rollup-plugin-json": "^2.1.0",
-    "rollup-watch": "^2.5.0",
+    "rollup-watch": "^4.3.1",
     "source-map": "^0.5.6",
     "ssri": "^4.1.2",
-    "uglify-js": "~2.7.3"
+    "uglify-js": "~3.0.26"
   },
   "main": "dist/leaflet-src.js",
   "style": "dist/leaflet.css",
diff --git a/spec/karma.conf.js b/spec/karma.conf.js
index 6bd405f3..01e828bf 100644
--- a/spec/karma.conf.js
+++ b/spec/karma.conf.js
@@ -25,7 +25,7 @@ module.exports = function (config) {
 		basePath: '../',
 
 		plugins: [
-			'karma-rollup-plugin',
+			'karma-rollup-preprocessor',
 			'karma-mocha',
 			'karma-coverage',
 			'karma-phantomjs-launcher',
diff --git a/spec/suites/core/GeneralSpec.js b/spec/suites/core/GeneralSpec.js
index c955e0ab..561d5e54 100644
--- a/spec/suites/core/GeneralSpec.js
+++ b/spec/suites/core/GeneralSpec.js
@@ -8,4 +8,10 @@ describe('General', function () {
 
 		expect(L.noConflict()).to.eql(leaflet);
 	});
+
+	describe('namespace extension', function () {
+		L.Util.foo = 'bar';
+
+		expect(L.Util.foo).to.eql('bar');
+	});
 });
diff --git a/src/Leaflet.js b/src/Leaflet.js
index 70a067dc..1337f07e 100644
--- a/src/Leaflet.js
+++ b/src/Leaflet.js
@@ -33,3 +33,6 @@ export function noConflict() {
 
 // Always export us to window global (see #2364)
 window.L = exports;
+
+import {freeze} from './core/Util';
+Object.freeze = freeze;
diff --git a/src/core/Util.js b/src/core/Util.js
index e1d02890..12ea3b1b 100644
--- a/src/core/Util.js
+++ b/src/core/Util.js
@@ -4,6 +4,9 @@
  * Various utility functions, used by Leaflet internally.
  */
 
+export var freeze = Object.freeze;
+Object.freeze = function (obj) { return obj; };
+
 // @function extend(dest: Object, src?: Object): Object
 // Merges the properties of the `src` object (or multiple objects) into `dest` object and returns the latter. Has an `L.extend` shortcut.
 export function extend(dest) {