Ignore upcoming experimental_member_use warnings. (#175969)

In an upcoming Dart SDK change
(https://dart-review.googlesource.com/c/sdk/+/450970), I intend to add
logic to the analyzer for generating a warning if an API marked
`@experimental` is used. This will allow experimental analyzer features
to be developed without creating a risk of breaking changes downstream.

It looks like there are two existing uses of an experimental API in the
flutter codebase: the method `XmlDocument.xpath`. This is probably safe,
since Flutter pins its use of `package:xml` to an exact version. But to
avoid breaking buildbots when the new analyzer logic rolls out, we need
to keep Flutter warning-clean.

So this PR proactively adds `// ignore:` comments to suppress the
upcoming warnings.

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packages/flutter_tools/lib/src/ios/xcode_debug.dart

@@ -410,6 +410,7 @@ class XcodeDebug {
     final String schemeXml = schemeFile.readAsStringSync();
     try {
       final document = XmlDocument.parse(schemeXml);
+      // ignore: experimental_member_use
       final Iterable<XmlNode> nodes = document.xpath('/Scheme/LaunchAction');
       if (nodes.isEmpty) {
         _logger.printError('Failed to find LaunchAction for the Scheme in ${schemeFile.path}.');

packages/flutter_tools/lib/src/migrations/lldb_init_migration.dart

@@ -249,6 +249,7 @@ selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
     required XmlDocument document,
     required SchemeInfo schemeInfo,
   }) {
+    // ignore: experimental_member_use
     final Iterable<XmlNode> nodes = document.xpath('/Scheme/$action');
     if (nodes.isEmpty) {
       logger.printTrace('Failed to find $action for the Scheme in ${schemeInfo.schemeFile.path}.');