Rewrite the AMP runtime library so that extensions loaded by the Shadow embed code will be loaded from localhost, not from the CDN

patch.patch

diff --git a/src/service/extensions-impl.js b/src/service/extensions-impl.js
index e6116b8ec..d40ea8ad8 100644
--- a/src/service/extensions-impl.js
+++ b/src/service/extensions-impl.js
@@ -556,6 +556,7 @@ export class Extensions {
     if (this.isExtensionScriptRequired_(extensionId, holder)) {
       const scriptElement =
           this.createExtensionScript_(extensionId, opt_extensionVersion);
+      scriptElement.src = scriptElement.src.replace(/https:\/\/cdn.ampproject.org\/rtv\/\d+\//, 'http://localhost:8000/dist/');
       this.win.document.head.appendChild(scriptElement);
       holder.scriptPresent = true;
     }
diff --git a/src/service/xhr-impl.js b/src/service/xhr-impl.js
index 3ed4c0cb6..71c197bb1 100644
--- a/src/service/xhr-impl.js
+++ b/src/service/xhr-impl.js
@@ -157,6 +157,9 @@ export class Xhr {
           if (init.responseType == 'document') {
             return fetchPolyfill(input, init);
           }
+          if (arguments.length && arguments[0].endsWith('/ww.js')) {
+            arguments[0] = 'http://localhost:8000/dist/ww.js';
+          }
           return (this.win.fetch || fetchPolyfill).apply(null, arguments);
         });
   }