lint: Directly ignore use_build_context_synchronously, where needed

This workaround recently stopped working: this inverted form of
early return no longer persuades the lint rule not to complain
when we go on to use `context`.

Not sure exactly what change caused that; it happened with a new
Flutter version from master/main in recent weeks, presumably due
to a new Dart SDK version rolling through there.

Since the workaround is no longer effective, just switch to the
straightforward early return, and use a lint-ignore comment
pointing at the underlying issue that causes the lint rule to
complain there:
  https://github.com/dart-lang/linter/issues/4007

That issue was recently fixed in the linter repo's main branch.
The fix rolled yesterday into the Dart SDK:
  dart-lang/sdk@f09ce66

but that hasn't quite yet rolled through to Flutter.
Once it does, we'll be able to cut these.

Author: gnprice

lib/widgets/clipboard.dart

@@ -19,8 +19,9 @@ void copyWithPopup({
   await Clipboard.setData(data);
   final deviceInfo = await DeviceInfoPlugin().deviceInfo;
 
-  if (context.mounted) {} // https://github.com/dart-lang/linter/issues/4007
-  else {
+  // https://github.com/dart-lang/linter/issues/4007
+  // ignore: use_build_context_synchronously
+  if (!context.mounted) {
     return;
   }
 

lib/widgets/compose_box.dart

@@ -306,8 +306,9 @@ abstract class _AttachUploadsButton extends StatelessWidget {
       return; // Nothing to do (getFiles handles user feedback)
     }
 
-    if (context.mounted) {} // https://github.com/dart-lang/linter/issues/4007
-    else {
+    // https://github.com/dart-lang/linter/issues/4007
+    // ignore: use_build_context_synchronously
+    if (!context.mounted) {
       return;
     }
 

lib/widgets/login.dart

@@ -49,8 +49,9 @@ class _AddAccountPageState extends State<AddAccountPage> {
 
     // TODO(#35): show feedback that we're working, while fetching server settings
     final serverSettings = await getServerSettings(realmUrl: url);
-    if (context.mounted) {} // https://github.com/dart-lang/linter/issues/4007
-    else {
+    // https://github.com/dart-lang/linter/issues/4007
+    // ignore: use_build_context_synchronously
+    if (!context.mounted) {
       return;
     }
 
@@ -149,8 +150,9 @@ class _EmailPasswordLoginPageState extends State<EmailPasswordLoginPage> {
 
       // TODO(server-7): Rely on user_id from fetchApiKey.
       final int userId = result.userId ?? await _getUserId(result);
-      if (context.mounted) {} // https://github.com/dart-lang/linter/issues/4007
-      else {
+      // https://github.com/dart-lang/linter/issues/4007
+      // ignore: use_build_context_synchronously
+      if (!context.mounted) {
         return;
       }
 
@@ -165,8 +167,9 @@ class _EmailPasswordLoginPageState extends State<EmailPasswordLoginPage> {
         zulipVersion: widget.serverSettings.zulipVersion,
         zulipMergeBase: Value(widget.serverSettings.zulipMergeBase),
       ));
-      if (context.mounted) {} // https://github.com/dart-lang/linter/issues/4007
-      else {
+      // https://github.com/dart-lang/linter/issues/4007
+      // ignore: use_build_context_synchronously
+      if (!context.mounted) {
         return;
       }