Call _isLoopback as a last resort (flutter/engine#28576)

engine/src/flutter/lib/ui/hooks.dart

@@ -249,20 +249,22 @@ bool _isLoopback(String host) {
 // ignore: unused_element
 void Function(Uri) _getHttpConnectionHookClosure(bool mayInsecurelyConnectToAllDomains) {
   return (Uri uri) {
-      if (_isLoopback(uri.host)) {
-        return;
-      }
       final dynamic zoneOverride = Zone.current[#flutter.io.allow_http];
       if (zoneOverride == true) {
         return;
       }
       if (zoneOverride == false && uri.isScheme('http')) {
-        // Going to throw
+        // Going to _isLoopback check before throwing
       } else if (mayInsecurelyConnectToAllDomains || uri.isScheme('https')) {
         // In absence of zone override, if engine setting allows the connection
         // or if connection is to `https`, allow the connection.
         return;
       }
+      // Loopback connections are always allowed
+      // Check at last resort to avoid debug annoyance of try/on ArgumentError
+      if (_isLoopback(uri.host)) {
+        return;
+      }
       throw UnsupportedError(
         'Non-https connection "$uri" is not supported by the platform. '
         'Refer to https://flutter.dev/docs/release/breaking-changes/network-policy-ios-android.');

engine/src/flutter/testing/dart/http_disallow_http_connections_test.dart

@@ -52,7 +52,7 @@ Future<bool> _supportsIPv6() async {
 }
 
 void main() {
-  test('testWithHostname', () async {
+  test('testWithLocalIP', () async {
     await bindServerAndTest(await getLocalHostIP(), (HttpClient httpClient, Uri httpUri) async {
       asyncExpectThrows<UnsupportedError>(
           () async =>  httpClient.getUrl(httpUri));
@@ -67,6 +67,30 @@ void main() {
     });
   });
 
+  test('testWithHostname', () async {
+    await bindServerAndTest(Platform.localHostname, (HttpClient httpClient, Uri httpUri) async {
+      asyncExpectThrows<UnsupportedError>(
+          () async =>  httpClient.getUrl(httpUri));
+
+      final _MockZoneValue mockFoo = _MockZoneValue('foo');
+      asyncExpectThrows<UnsupportedError>(
+          () async => runZoned(() => httpClient.getUrl(httpUri),
+            zoneValues: <dynamic, dynamic>{#flutter.io.allow_http: mockFoo}));
+      expect(mockFoo.checked, isTrue);
+
+      final _MockZoneValue mockFalse = _MockZoneValue(false);
+      asyncExpectThrows<UnsupportedError>(
+          () async => runZoned(() => httpClient.getUrl(httpUri),
+            zoneValues: <dynamic, dynamic>{#flutter.io.allow_http: mockFalse}));
+      expect(mockFalse.checked, isTrue);
+
+      final _MockZoneValue mockTrue = _MockZoneValue(true);
+      await runZoned(() => httpClient.getUrl(httpUri),
+        zoneValues: <dynamic, dynamic>{#flutter.io.allow_http: mockTrue});
+      expect(mockFalse.checked, isTrue);
+    });
+  });
+
   test('testWithLoopback', () async {
     await bindServerAndTest('127.0.0.1', (HttpClient httpClient, Uri uri) async {
       await httpClient.getUrl(Uri.parse('http://localhost:${uri.port}'));
@@ -82,3 +106,27 @@ void main() {
     }
   });
 }
+
+class _MockZoneValue {
+  _MockZoneValue(this._value);
+
+  Object? _value;
+  bool _falseChecked = false;
+  bool _trueChecked = false;
+
+  @override
+  bool operator ==(Object o) {
+    if(o == true) {
+      _trueChecked = true;
+    }
+    if(o == false) {
+      _falseChecked = true;
+    }
+    return _value == o;
+  }
+
+  bool get checked => _falseChecked && _trueChecked;
+
+  @override
+  int get hashCode => _value.hashCode;
+}