Dart: Wait to handle events on a Stub until it makes sense to do it.

mojo/dart/packages/mojo_services/lib/mojo/media/media_sink.mojom.dart

Index: mojo/dart/packages/mojo_services/lib/mojo/media/media_sink.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/mojo/media/media_sink.mojom.dart b/mojo/dart/packages/mojo_services/lib/mojo/media/media_sink.mojom.dart
index be2f76b1dac21828e05055cecfb100a9b3766497..117e787faf627acdb508a61050fdca5d7c73d18f 100644
--- a/mojo/dart/packages/mojo_services/lib/mojo/media/media_sink.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/mojo/media/media_sink.mojom.dart
@@ -630,14 +630,19 @@ class MediaSinkProxy implements bindings.ProxyBase {
 
 
 class MediaSinkStub extends bindings.Stub {
-  MediaSink _impl = null;
+  MediaSink _impl;
 
   MediaSinkStub.fromEndpoint(
-      core.MojoMessagePipeEndpoint endpoint, [this._impl])
-      : super.fromEndpoint(endpoint);
+      core.MojoMessagePipeEndpoint endpoint, [MediaSink impl])
+      : super.fromEndpoint(endpoint, autoBegin: impl != null) {
+    _impl = impl;
+  }
 
-  MediaSinkStub.fromHandle(core.MojoHandle handle, [this._impl])
-      : super.fromHandle(handle);
+  MediaSinkStub.fromHandle(
+      core.MojoHandle handle, [MediaSink impl])
+      : super.fromHandle(handle, autoBegin: impl != null) {
+    _impl = impl;
+  }
 
   MediaSinkStub.unbound() : super.unbound();
 
@@ -661,7 +666,9 @@ class MediaSinkStub extends bindings.Stub {
                                                           0,
                                                           message);
     }
-    assert(_impl != null);
+    if (_impl == null) {
+      throw new core.MojoApiError("$this has no implementation set");
+    }
     switch (message.header.type) {
       case _mediaSinkMethodGetConsumerName:
         var params = _MediaSinkGetConsumerParams.deserialize(
@@ -705,10 +712,23 @@ class MediaSinkStub extends bindings.Stub {
 
   MediaSink get impl => _impl;
   set impl(MediaSink d) {
-    assert(_impl == null);
+    if (d == null) {
+      throw new core.MojoApiError("$this: Cannot set a null implementation");
+    }
+    if (isBound && (_impl == null)) {
+      beginHandlingEvents();
+    }
     _impl = d;
   }
 
+  @override
+  void bind(core.MojoMessagePipeEndpoint endpoint) {
+    super.bind(endpoint);
+    if (!isOpen && (_impl != null)) {
+      beginHandlingEvents();
+    }
+  }
+
   String toString() {
     var superString = super.toString();
     return "MediaSinkStub($superString)";