Do not report context.mounted as a use of BuildContext (#4312)

Author: srawlins

lib/src/rules/use_build_context_synchronously.dart

@@ -106,6 +106,8 @@ class _AwaitVisitor extends RecursiveAstVisitor {
 }
 
 class _Visitor extends SimpleAstVisitor {
+  static const mountedName = 'mounted';
+
   final LintRule rule;
 
   _Visitor(this.rule);
@@ -238,7 +240,7 @@ class _Visitor extends SimpleAstVisitor {
           check = check.identifier;
         }
         if (check is SimpleIdentifier) {
-          return check.name == 'mounted';
+          return check.name == mountedName;
         }
         if (check is PrefixExpression) {
           // (condition || !mounted)
@@ -298,6 +300,11 @@ class _Visitor extends SimpleAstVisitor {
 
   @override
   visitPrefixedIdentifier(PrefixedIdentifier node) {
+    if (node.identifier.name == mountedName) {
+      // Accessing `context.mounted` does not count as a "use" of a
+      // `BuildContext` which needs to be guarded by a mounted check.
+      return;
+    }
     // Getter access.
     if (isBuildContext(node.prefix.staticType, skipNullable: true)) {
       check(node);

test/rules/use_build_context_synchronously_test.dart

@@ -72,6 +72,40 @@ bool get c => true;
 ''');
   }
 
+  test_awaitBeforeIf_mountedExitGuardInIf_beforeReferenceToContext2() async {
+    // Await, then a proper "exit if not mounted" guard in an if-condition,
+    // then use of BuildContext, is OK.
+    await assertNoDiagnostics(r'''
+import 'package:flutter/widgets.dart';
+
+void foo(BuildContext context) async {
+  await f();
+  if (!context.mounted) return;
+  Navigator.of(context);
+}
+
+Future<void> f() async {}
+''');
+  }
+
+  test_awaitBeforeIf_mountedExitGuardInIf_beforeReferenceToContext3() async {
+    // Await, then a proper "exit if not mounted" guard in an if-condition,
+    // then use of BuildContext, is OK.
+    await assertNoDiagnostics(r'''
+import 'package:flutter/widgets.dart';
+
+void foo(BuildContext context) async {
+  await f();
+  if (!context.mounted) {
+    return;
+  }
+  Navigator.of(context);
+}
+
+Future<void> f() async {}
+''');
+  }
+
   test_awaitBeforeIf_mountedGuardInIf1() async {
     // Await, then a proper "if mounted" guard in an if-condition, then use of
     // BuildContext in the if-body, is OK.