Update image_list example to use https instead of http (flutter#39124)

* Update image_list test to use locally set up https server to validate performance of image loading over https(rather than http) protocol. Https requires more back-and-forward communication before image bytes are sent to the client, shows clear signs of throughput affected by how busy flutter UI isolate is.

examples/image_list/lib/main.dart

@@ -4,6 +4,7 @@
 
 import 'dart:async';
 import 'dart:io';
+import 'dart:math';
 import 'dart:typed_data';
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart' show rootBundle;
@@ -14,19 +15,98 @@ import 'package:flutter/services.dart';
 /// image and prints how long the loading took.
 ///
 /// This is used in [$FH/flutter/devicelab/bin/tasks/image_list_reported_duration.dart] test.
+///
+String certificate = '''
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----
+''';
+
+String privateKey = '''
+-----BEGIN PRIVATE KEY-----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+-----END PRIVATE KEY-----
+''';
+
+class MyHttpOverrides extends HttpOverrides {
+  @override
+  HttpClient createHttpClient(SecurityContext context) {
+    return super.createHttpClient(
+        (context ?? SecurityContext())..setTrustedCertificatesBytes(certificate.codeUnits)
+    );
+  }
+}
+
 Future<void> main() async {
+  HttpOverrides.global = MyHttpOverrides();
+
+  final SecurityContext serverContext = SecurityContext()
+    ..useCertificateChainBytes(certificate.codeUnits)
+    ..usePrivateKeyBytes(privateKey.codeUnits);
+
   final HttpServer httpServer =
-      await HttpServer.bind(InternetAddress.anyIPv6, 0);
+      await HttpServer.bindSecure('localhost', 0, serverContext);
   final int port = httpServer.port;
   print('Listening on port $port.');
 
   // Initializes bindings before using any platform channels.
   WidgetsFlutterBinding.ensureInitialized();
   final ByteData byteData = await rootBundle.load('images/coast.jpg');
-  final Uint8List bytes = byteData.buffer
-      .asUint8List(byteData.offsetInBytes, byteData.lengthInBytes);
   httpServer.listen((HttpRequest request) async {
-    request.response.add(bytes);
+    const int chunk_size = 2048;
+    int offset = byteData.offsetInBytes;
+    while (offset < byteData.lengthInBytes) {
+      final int length = min(byteData.lengthInBytes - offset, chunk_size);
+      final Uint8List bytes = byteData.buffer.asUint8List(offset, length);
+      offset += length;
+      request.response.add(bytes);
+      // Let other isolates and microtasks to run.
+      await Future<void>.delayed(const Duration());
+    }
     request.response.close();
   });
 
@@ -73,7 +153,7 @@ class _MyHomePageState extends State<MyHomePage> with TickerProviderStateMixin {
 
   Widget createImage(final int index, final Completer<bool> completer) {
     return Image.network(
-        'http://localhost:${widget.port}/${_counter * IMAGES + index}',
+        'https://localhost:${widget.port}/${_counter * IMAGES + index}',
         frameBuilder: (
           BuildContext context,
           Widget child,