[dart:html] Change AudioNode.connect to be nullable

AudioNode.connect can return undefined in the case where
destination is an AudioParam. Note this is not a breaking
change since it's a private method. This unbreaks users
who use native null assertions.

Change-Id: I517863798b0ebe4859d195dcadc2696716ebc8a8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/237500
Commit-Queue: Srujan Gaddam <srujzs@google.com>
Reviewed-by: Riley Porter <rileyporter@google.com>

sdk/lib/web_audio/dart2js/web_audio_dart2js.dart

@@ -386,7 +386,7 @@ class AudioNode extends EventTarget {
   int? get numberOfOutputs native;
 
   @JSName('connect')
-  AudioNode _connect(destination, [int? output, int? input]) native;
+  AudioNode? _connect(destination, [int? output, int? input]) native;
 
   void disconnect([destination_OR_output, int? output, int? input]) native;
 

tools/dom/idl/dart/dart.idl

@@ -450,6 +450,15 @@ interface AudioContext {
     [DartSuppress] DelayNode createDelayNode(optional double maxDelayTime);
 };
 
+[DartSupplemental]
+interface AudioNode : EventTarget {
+    [DartSuppress] AudioNode connect(AudioNode destination, optional unsigned long output = 0, optional unsigned long input = 0);
+    [DartSuppress] void connect(AudioParam destination, optional unsigned long output = 0);
+    AudioNode? connect(AudioNode destination, optional unsigned long output = 0, optional unsigned long input = 0);
+    // Scripts don't unify return types, so this is changed from void to do so.
+    AudioNode? connect(AudioParam destination, optional unsigned long output = 0);
+};
+
 [DartSupplemental]
 interface HTMLInputElement {
     [DartSuppress] attribute boolean webkitSpeech;