platform: Use Flutter "target" platform, not actual Platform

This makes it possible to test this sort of logic.

See docs for [defaultTargetPlatform]:
  https://api.flutter.dev/flutter/foundation/defaultTargetPlatform.html

Author: gnprice

lib/widgets/message_list.dart

@@ -1,4 +1,3 @@
-import 'dart:io' show Platform;
 import 'package:flutter/material.dart';
 import 'package:intl/intl.dart';
 
@@ -172,12 +171,13 @@ class _MessageListState extends State<MessageList> {
       // TODO: Offer `ScrollViewKeyboardDismissBehavior.interactive` (or
       //   similar) if that is ever offered:
       //     https://github.com/flutter/flutter/issues/57609#issuecomment-1355340849
-      keyboardDismissBehavior: Platform.isIOS
+      keyboardDismissBehavior: switch (Theme.of(context).platform) {
         // This seems to offer the only built-in way to close the keyboard
         // on iOS. It's not ideal; see TODO above.
-        ? ScrollViewKeyboardDismissBehavior.onDrag
+        TargetPlatform.iOS => ScrollViewKeyboardDismissBehavior.onDrag,
         // The Android keyboard seems to have a built-in close button.
-        : ScrollViewKeyboardDismissBehavior.manual,
+        _ => ScrollViewKeyboardDismissBehavior.manual,
+      },
 
       itemCount: length,
       // Setting reverse: true means the scroll starts at the bottom.

lib/widgets/text.dart

@@ -19,6 +19,7 @@ final TextStyle kMonospaceTextStyle = TextStyle(
 
   // Oddly, iOS doesn't handle 'monospace':
   //   https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/flutter.3A.20monospace.20font.20fallback/near/1570622
+  // TODO use Theme.of, not Platform, so that this is testable (needs a BuildContext)
   fontFamilyFallback: Platform.isIOS ? ['Menlo', 'Courier'] : ['monospace'],
 
   inherit: true,