compose [nfc]: Simplify choose-body logic by switching on controller

Author: chrisbobbe

lib/widgets/compose_box.dart

@@ -1356,24 +1356,17 @@ class _ComposeBoxState extends State<ComposeBox> implements ComposeBoxState {
       //         ZulipLocalizations.of(context).errorSendMessageTimeout);
       //     }
       final narrow = widget.narrow;
-      switch (narrow) {
-        case ChannelNarrow():
-          assert(_controller is StreamComposeBoxController);
-          body = _StreamComposeBoxBody(controller: (_controller as StreamComposeBoxController),
-            narrow: narrow);
-        case TopicNarrow():
-          assert(_controller is FixedDestinationComposeBoxController);
-          body = _FixedDestinationComposeBoxBody(controller: (_controller as FixedDestinationComposeBoxController),
-            narrow: narrow);
-        case DmNarrow():
-          assert(_controller is FixedDestinationComposeBoxController);
-          body = _FixedDestinationComposeBoxBody(controller: (_controller as FixedDestinationComposeBoxController),
-            narrow: narrow);
-        case CombinedFeedNarrow():
-        case MentionsNarrow():
-        case StarredMessagesNarrow():
-          assert(false);
-          body = null;
+      switch (_controller) {
+        case StreamComposeBoxController(): {
+          assert(narrow is ChannelNarrow);
+          body = _StreamComposeBoxBody(controller: _controller,
+            narrow: (narrow as ChannelNarrow));
+        }
+        case FixedDestinationComposeBoxController(): {
+          assert(narrow is SendableNarrow);
+          body = _FixedDestinationComposeBoxBody(controller: _controller,
+            narrow: (narrow as SendableNarrow));
+        }
       }
     }