feat: support browse files on web (#283)

ui/flutter/lib/util/browser_download/browser_download.dart

@@ -0,0 +1,4 @@
+import 'browser_download_stub.dart'
+    if (dart.library.html) 'entry/browser_download_browser.dart';
+
+void download(String url, String name) => doDownload(url, name);

ui/flutter/lib/util/browser_download/browser_download_stub.dart

@@ -0,0 +1 @@
+void doDownload(String url, String name) => throw UnimplementedError();

ui/flutter/lib/util/browser_download/entry/browser_download_browser.dart

@@ -0,0 +1,9 @@
+// ignore: avoid_web_libraries_in_flutter
+import 'dart:html' as html;
+
+void doDownload(String url, String name) {
+  final anchorElement = html.AnchorElement(href: url);
+  anchorElement.download = name;
+  anchorElement.target = '_blank';
+  anchorElement.click();
+}