[nfc]: Remove unnecessary imports of dart:ui

Upstream Flutter (in flutter/flutter@c6741614041 )
changed `flutter/material.dart` to additionally export
many types from `dart:ui`. This caused a lot of our
imports to trigger an analysis warning of
`unnecessary_import`.

Author: sirpengi

lib/widgets/message_list.dart

@@ -1,5 +1,4 @@
 import 'dart:math';
-import 'dart:ui';
 
 import 'package:collection/collection.dart';
 import 'package:flutter/material.dart';

lib/widgets/text.dart

@@ -1,5 +1,4 @@
 import 'dart:io';
-import 'dart:ui';
 import 'package:flutter/widgets.dart';
 
 /// A mergeable [TextStyle] with 'Source Code Pro' and platform-aware fallbacks.

lib/widgets/unread_count_badge.dart

@@ -1,4 +1,3 @@
-import 'dart:ui';
 
 import 'package:flutter/material.dart';
 

test/flutter_checks.dart

@@ -1,8 +1,6 @@
 /// `package:checks`-related extensions for the Flutter framework.
 library;
 
-import 'dart:ui';
-
 import 'package:checks/checks.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter/widgets.dart';

test/widgets/text_test.dart

@@ -1,4 +1,3 @@
-import 'dart:ui';
 
 import 'package:checks/checks.dart';
 import 'package:flutter/material.dart';