Fix a few strong mode errors in dart:html

tools/dom/scripts/htmleventgenerator.py

Index: tools/dom/scripts/htmleventgenerator.py
diff --git a/tools/dom/scripts/htmleventgenerator.py b/tools/dom/scripts/htmleventgenerator.py
index 6e83fd3f1b829fb5bec95c2e5f96069548267395..9c1928a57652a4efdc4280b7bc53acdf075f8dc8 100644
--- a/tools/dom/scripts/htmleventgenerator.py
+++ b/tools/dom/scripts/htmleventgenerator.py
@@ -123,6 +123,7 @@ _html_event_types = monitored.Dict('htmleventgenerator._html_event_types', {
   'ApplicationCache.obsolete': ('obsolete', 'Event'),
   'ApplicationCache.progress': ('progress', 'ProgressEvent'),
   'ApplicationCache.updateready': ('updateReady', 'Event'),
+  'CompositorWorker.error': ('error', 'ErrorEvent'),
   'Document.readystatechange': ('readyStateChange', 'Event'),
   'Document.securitypolicyviolation': ('securityPolicyViolation', 'SecurityPolicyViolationEvent'),
   'Document.selectionchange': ('selectionChange', 'Event'),
@@ -190,6 +191,7 @@ _html_event_types = monitored.Dict('htmleventgenerator._html_event_types', {
   'RTCPeerConnection.removestream': ('removeStream', 'MediaStreamEvent'),
   'RTCPeerConnection.signalingstatechange': ('signalingStateChange', 'Event'),
   'ScriptProcessorNode.audioprocess': ('audioProcess', 'AudioProcessingEvent'),
+  'SharedWorker.error': ('error', 'ErrorEvent'),
   'SharedWorkerGlobalScope.connect': ('connect', 'Event'),
   'SpeechRecognition.audioend': ('audioEnd', 'Event'),
   'SpeechRecognition.audiostart': ('audioStart', 'Event'),
@@ -221,6 +223,7 @@ _html_event_types = monitored.Dict('htmleventgenerator._html_event_types', {
   'Window.pageshow': ('pageShow', 'Event'),
   'Window.progress': ('progress', 'Event'),
   'Window.webkittransitionend': ('webkitTransitionEnd', 'TransitionEvent'),
+  'Worker.error': ('error', 'ErrorEvent'),
   'XMLHttpRequestEventTarget.abort': ('abort', 'ProgressEvent'),
   'XMLHttpRequestEventTarget.error': ('error', 'ProgressEvent'),
   'XMLHttpRequestEventTarget.load': ('load', 'ProgressEvent'),