First try at removing ? from HTML.

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index 917f711df006e5cb83a7c5c75dcb12b45279670f..591ccd65e7a4c1d8f2365701ba29e6fb89b4ba86 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -18080,7 +18080,7 @@ class RtcPeerConnection extends EventTarget native "RTCPeerConnection" {
   factory RtcPeerConnection(Map rtcIceServers, [Map mediaConstraints]) {
     var constructorName = JS('RtcPeerConnection', 'window[#]',
         '${Device.propertyPrefix}RTCPeerConnection');
-    if (?mediaConstraints) {
+    if (mediaConstraints != null) {
       return JS('RtcPeerConnection', 'new #(#,#)', constructorName,
           convertDartToNative_SerializedScriptValue(rtcIceServers),
           convertDartToNative_SerializedScriptValue(mediaConstraints));
@@ -22611,7 +22611,7 @@ class Window extends EventTarget implements WindowBase native "Window,DOMWindow"
   @SupportedBrowser(SupportedBrowser.IE, '10.0')
   @Experimental
   IdbFactory get indexedDB =>
-      JS('IdbFactory|Null',
+      JS('IdbFactory|Null',  // If not supported, returns `null`.
          '#.indexedDB || #.webkitIndexedDB || #.mozIndexedDB',
          this, this, this);