diff -r 000000000000 -r 6474c204b198 dom/apps/src/ScriptPreloader.jsm
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/dom/apps/src/ScriptPreloader.jsm	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,76 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const Cu = Components.utils;
+const Ci = Components.interfaces;
+
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+Cu.import("resource://gre/modules/Services.jsm");
+Cu.import("resource://gre/modules/Promise.jsm");
+
+this.EXPORTED_SYMBOLS = ["ScriptPreloader"];
+
+function debug(aMsg) {
+  //dump("--*-- ScriptPreloader: " + aMsg + "\n");
+}
+
+this.ScriptPreloader = {
+#ifdef MOZ_B2G
+  _enabled: true,
+#else
+  _enabled: false,
+#endif
+
+  preload: function(aApp, aManifest) {
+    debug("Preloading " + aApp.origin);
+    let deferred = Promise.defer();
+
+    if (!this._enabled) {
+      deferred.resolve();
+      return deferred.promise;
+    }
+
+    if (aManifest.precompile &&
+        Array.isArray(aManifest.precompile) &&
+        aManifest.precompile.length > 0) {
+      let origin = Services.io.newURI(aApp.origin, null, null);
+      let toLoad = aManifest.precompile.length;
+      let principal =
+        Services.scriptSecurityManager
+                .getAppCodebasePrincipal(origin, aApp.localId, false);
+
+      aManifest.precompile.forEach((aPath) => {
+        let uri = Services.io.newURI(aPath, null, origin);
+        debug("Script to compile: " + uri.spec);
+        try {
+          Services.scriptloader.precompileScript(uri, principal,
+            (aSubject, aTopic, aData) => {
+              let uri = aSubject.QueryInterface(Ci.nsIURI);
+              debug("Done compiling " + uri.spec);
+
+              toLoad--;
+              if (toLoad == 0) {
+                deferred.resolve();
+              }
+            });
+        } catch (e) {
+          // Resolve the promise if precompileScript throws.
+          deferred.resolve();
+        }
+      });
+    } else {
+      // The precompile field is not an array, let the developer know.
+      // We don't want to have to enable debug for that to show up.
+      if (aManifest.precompile) {
+        Cu.reportError("ASM.JS compilation failed: the 'precompile' manifest " +
+                       "property should be an array of script uris.\n");
+      }
+      deferred.resolve();
+    }
+
+    return deferred.promise;
+  }
+}