Do not use window global in HMR runtime (#529)

The name of the global object may differ based on the context in which the
script is running. For example, web workers use "self" instead of "window". As
location is a property of the global, it can be referenced directly as a global
variable, which avoids having to deal with this naming inconsistency.

Fixes #528

src/builtins/hmr-runtime.js

@@ -15,7 +15,7 @@ function Module() {
 module.bundle.Module = Module;
 
 if (!module.bundle.parent && typeof WebSocket !== 'undefined') {
-  var ws = new WebSocket('ws://' + window.location.hostname + ':{{HMR_PORT}}/');
+  var ws = new WebSocket('ws://' + location.hostname + ':{{HMR_PORT}}/');
   ws.onmessage = function(event) {
     var data = JSON.parse(event.data);
 
@@ -34,7 +34,7 @@ if (!module.bundle.parent && typeof WebSocket !== 'undefined') {
     if (data.type === 'reload') {
       ws.close();
       ws.onclose = function () {
-        window.location.reload();
+        location.reload();
       }
     }