Make browser tests all run from a server instead of the local filesystem.

samples/swarm/DataSource.dart

Index: samples/swarm/DataSource.dart
diff --git a/samples/swarm/DataSource.dart b/samples/swarm/DataSource.dart
index 8c7d0e82fe8c2af76a5d3f4d7f5a926ce74588d9..e471567e19311fc8613c2191b40123bbb781dc75 100644
--- a/samples/swarm/DataSource.dart
+++ b/samples/swarm/DataSource.dart
@@ -56,8 +56,9 @@ class Sections extends Collection<Section> {
     callback(new Sections(sections));
   }
 
-  static void initializeFromUrl(void callback(Sections sections)) {
-    if (Sections.runningFromFile) {
+  static void initializeFromUrl(bool useCannedData,
+                                void callback(Sections sections)) {
+    if (Sections.runningFromFile || useCannedData) {
       initializeFromData(CannedData.data['user.data'], callback);
     } else {
       // TODO(jmesserly): display an error if we fail here! Silent failure bad.