fixed #1650

lib/HotModuleReplacement.runtime.js

@@ -29,7 +29,10 @@ module.exports = function() {
 		};
 		for(var name in $require$) {
 			if(Object.prototype.hasOwnProperty.call($require$, name)) {
-				fn[name] = $require$[name];
+				Object.defineProperty(fn, name, {
+					get: function() { return $require$[name]; },
+					set: function(value) { $require$[name] = value; }
+				});
 			}
 		}
 		fn.e = function(chunkId) {

test/cases/runtime/issue-1650/file.js

@@ -0,0 +1 @@
+__webpack_public_path__ = "ok";

test/cases/runtime/issue-1650/index.js

@@ -0,0 +1,6 @@
+it("should be able to set the public path globally", function() {
+	var org = __webpack_public_path__;
+	require("./file");
+	__webpack_public_path__.should.be.eql("ok");
+	__webpack_public_path__ = org;
+});