[url_launcher] Ignore new unreachable_switch_default warning. (#7487)

The Dart analyzer will soon be changed so that if the `default` clause
of a `switch` statement is determined to be unreachable by the
exhaustiveness checker, a new warning of type
`unreachable_switch_default` will be issued. This parallels the behavior
of the existing `unreachable_switch_case` warning, which is issued
whenever a `case` clause of a `switch` statement is determined to be
unreachable.

In the vast majority of cases, the most reasonable way to address the
warning is to remove the unreachable `default` clause. However, in a few
rare cases, it makes sense to keep the `default` clause, because it's
intentionally future-proofing the code in case new possible values are
added to the enum being switched on.

Three of these rare cases crop up in flutter/packages. This change adds
`ignore` comments to avoid a spurious warning.

packages/url_launcher/url_launcher_android/lib/url_launcher_android.dart

@@ -89,7 +89,7 @@ class UrlLauncherAndroid extends UrlLauncherPlatform {
       case PreferredLaunchMode.platformDefault:
       // Intentionally treat any new values as platformDefault; see comment in
       // supportsMode.
-      // ignore: no_default_cases
+      // ignore: no_default_cases, unreachable_switch_default
       default:
         // By default, open web URLs in the application.
         inApp = url.startsWith('http:') || url.startsWith('https:');

packages/url_launcher/url_launcher_ios/lib/url_launcher_ios.dart

@@ -82,7 +82,7 @@ class UrlLauncherIOS extends UrlLauncherPlatform {
       // Intentionally treat any new values as platformDefault; support for any
       // new mode requires intentional opt-in, otherwise falling back is the
       // documented behavior.
-      // ignore: no_default_cases
+      // ignore: no_default_cases, unreachable_switch_default
       default:
         // By default, open web URLs in the application.
         inApp = url.startsWith('http:') || url.startsWith('https:');
@@ -110,7 +110,7 @@ class UrlLauncherIOS extends UrlLauncherPlatform {
       // Default is a desired behavior here since support for new modes is
       // always opt-in, and the enum lives in a different package, so silently
       // adding "false" for new values is the correct behavior.
-      // ignore: no_default_cases
+      // ignore: no_default_cases, unreachable_switch_default
       default:
         return false;
     }