Remove HTMLOutputElementCustom which is no longer needed.

Source/WebCore/bindings/dart/custom/DartHTMLMediaElementCustom.cpp

Index: Source/WebCore/bindings/dart/custom/DartHTMLMediaElementCustom.cpp
diff --git a/Source/WebCore/bindings/dart/custom/DartHTMLMediaElementCustom.cpp b/Source/WebCore/bindings/dart/custom/DartHTMLMediaElementCustom.cpp
index cb213cafefb1315439c0dbf1e5383679b8932b2e..2ac10971a95856c0be91c7632b55349aaa51b6f5 100644
--- a/Source/WebCore/bindings/dart/custom/DartHTMLMediaElementCustom.cpp
+++ b/Source/WebCore/bindings/dart/custom/DartHTMLMediaElementCustom.cpp
@@ -33,14 +33,33 @@
 
 #include "DartHTMLMediaElement.h"
 
+#include "DartMediaController.h"
+
 namespace WebCore {
 
 namespace DartHTMLMediaElementInternal {
 
-void controllerSetter(Dart_NativeArguments)
+void controllerSetter(Dart_NativeArguments args)
 {
-    // FIXME: proper implementation.
-    DART_UNIMPLEMENTED();
+    DartApiScope dartApiScope;
+    Dart_Handle exception = 0;
+    {
+        HTMLMediaElement* receiver = DartDOMWrapper::receiver<HTMLMediaElement>(args);
+        MediaController* controller = DartDOMWrapper::unwrapDartWrapper<DartMediaController>(Dart_GetNativeArgument(args, 1), exception);
+        if (exception)
+            goto fail;
+
+        // 4.8.10.11.2 Media controllers: controller attribute.
+        // On setting, it must first remove the element's mediagroup attribute, if any,
+        receiver->setMediaGroup(String());
+        // and then set the current media controller to the given value.
+        receiver->setController(controller);
+        return;
+    }
+
+fail:
+    Dart_ThrowException(exception);
+    ASSERT_NOT_REACHED();
 }
 
 }