CLJS-1114: browser REPL script loading race condition

the browser REPL cannot use document.write to load scripts after
page load as that would blow away the document.

However appending script element to document.body suffers from
race conditions due to asynchronous script loading when not
using document.write.

Add a load queue which enforces sequential load order.

Author: swannodette

src/cljs/clojure/browser/repl.cljs

@@ -109,6 +109,8 @@
       (js/setTimeout #(send-result connection url (wrap-message :ready "ready")) 50))
     (js/alert "No 'xpc' param provided to child iframe.")))
 
+(def load-queue nil)
+
 (defn connect
   "Connects to a REPL server from an HTML document. After the
   connection is made, the REPL will evaluate forms in the context of
@@ -135,14 +137,28 @@
       (set! (.-require__ js/goog) js/goog.require)
       ;; suppress useless Google Closure error about duplicate provides
       (set! (.-isProvided_ js/goog) (fn [name] false))
-      (set! (.-writeScriptTag_ js/goog)
+      (set! (.-writeScriptTag__ js/goog)
         (fn [src opt_sourceText]
           (.appendChild js/document.body
             (as-> (.createElement js/document "script") script
-              (doto script (aset "type" "text/javascript"))
+              (doto script
+                (aset "type" "text/javascript")
+                (aset "onload"
+                  (fn []
+                    (when load-queue
+                      (if (zero? (alength load-queue))
+                        (set! load-queue nil)
+                        (.apply js/goog.writeScriptTag__ nil (.shift load-queue)))))))
               (if (nil? opt_sourceText)
                 (doto script (aset "src" src))
                 (doto script (gdom/setTextContext opt_sourceText)))))))
+      (set! (.-writeScriptTag_ js/goog)
+        (fn [src opt_sourceText]
+          (if load-queue
+            (.push load-queue #js [src opt_sourceText])
+            (do
+              (set! load-queue #js [])
+              (js/goog.writeScriptTag__ src opt_sourceText)))))
       (set! (.-require js/goog)
         (fn [src reload]
           (when (= reload "reload-all")