First pass at cleaning up html lib warnings from the analyzer.

tools/dom/src/EventStreamProvider.dart

Index: tools/dom/src/EventStreamProvider.dart
diff --git a/tools/dom/src/EventStreamProvider.dart b/tools/dom/src/EventStreamProvider.dart
index 2e787bd5ff986025ed32044bed10ad67e2f1fbcc..85b4d78cb383a32b98838a3889b2c3c1c62129f1 100644
--- a/tools/dom/src/EventStreamProvider.dart
+++ b/tools/dom/src/EventStreamProvider.dart
@@ -223,13 +223,14 @@ class _EventStreamSubscription<T extends Event> extends StreamSubscription<T> {
     _tryResume();
   }
 
-  void cancel() {
-    if (_canceled) return;
+  Future cancel() {
+    if (_canceled) return null;
 
     _unlisten();
     // Clear out the target to indicate this is complete.
     _target = null;
     _onData = null;
+    return null;
   }
 
   bool get _canceled => _target == null;