Normalize http resourced paths before passing it further to the app

After deploying the new flex version of pub.dartlang.org travis builds
suddendly started to fail.

It seems that travis is using a special PUB_HOSTED_URL with a slash at
the end, which causes the travis builds's pub to make requests like:

  GET //api/packages/http

[email protected]

Review-Url: https://codereview.chromium.org/2754713003 .

Author: mkustermann

app/bin/server.dart

@@ -7,6 +7,7 @@ import 'package:gcloud/db.dart';
 import 'package:gcloud/service_scope.dart';
 import 'package:gcloud/storage.dart';
 import 'package:googleapis_auth/auth_io.dart' as auth;
+import 'package:path/path.dart' as path;
 import 'package:shelf/shelf.dart' as shelf;
 import 'package:shelf/shelf_io.dart' as shelf_io;
 
@@ -80,6 +81,7 @@ void main() {
 
               logger.info('Handling request: ${request.requestedUri} '
                           '(Using namespace "$namespace")');
+              request = sanitizeRequestedUri(request);
               return appHandler(request, apiHandler).catchError((error, s) {
                 logger.severe('Request handler failed', error, s);
                 return new shelf.Response.internalServerError();
@@ -98,3 +100,36 @@ void main() {
 
 /// Gets the namespace to use.
 String getCurrentNamespace() => '';
+
+shelf.Request sanitizeRequestedUri(shelf.Request request) {
+  final uri = request.requestedUri;
+  final resource = uri.path;
+  final normalizedResource = path.normalize(resource);
+
+  if (resource == normalizedResource) {
+    return request;
+  } else {
+    // With the new flex VMs we will get requests from the L7 load balancer which
+    // can contain [Uri]s with e.g. double slashes
+    //
+    //    -> e.g. https://pub.dartlang.org//api/packages/foo
+    //
+    // It seems that travis builds e.g. set PUB_HOSTED_URL to a URL with
+    // a slash at the end. The pub client will not remove it and instead
+    // directly try to request the URI:
+    //
+    //    GET //api/...
+    //
+    // :-/
+
+    final changedUri = uri.replace(path: normalizedResource);
+    return new shelf.Request(
+        request.method,
+        changedUri,
+        protocolVersion: request.protocolVersion,
+        headers: request.headers,
+        body: request.read(),
+        encoding: request.encoding,
+        context: request.context);
+  }
+}