[Material] Fix showDialog crasher caused by old contexts (#30754)

* Fix bug, add regression test.

* remove unnecessary space

packages/flutter/lib/src/material/dialog.dart

@@ -700,11 +700,12 @@ Future<T> showDialog<T>({
 }) {
   assert(child == null || builder == null);
   assert(debugCheckHasMaterialLocalizations(context));
+
+  final ThemeData theme = Theme.of(context, shadowThemeOnly: true);
   return showGeneralDialog(
     context: context,
     pageBuilder: (BuildContext buildContext, Animation<double> animation, Animation<double> secondaryAnimation) {
-      final ThemeData theme = Theme.of(context, shadowThemeOnly: true);
-      final Widget pageChild =  child ?? Builder(builder: builder);
+      final Widget pageChild = child ?? Builder(builder: builder);
       return SafeArea(
         child: Builder(
           builder: (BuildContext context) {

packages/flutter/test/material/dialog_test.dart

@@ -607,4 +607,42 @@ void main() {
     expect(find.text('0'), findsNothing);
     expect(find.text('1'), findsNothing);
   });
+
+  // Regression test for https://github.com/flutter/flutter/issues/28505.
+  testWidgets('showDialog only gets Theme from context on the first call', (WidgetTester tester) async {
+    Widget buildFrame(Key builderKey) {
+      return MaterialApp(
+        home: Center(
+          child: Builder(
+            key: builderKey,
+            builder: (BuildContext outerContext) {
+              return RaisedButton(
+                onPressed: () {
+                  showDialog<void>(
+                    context: outerContext,
+                    builder: (BuildContext innerContext) {
+                      return const AlertDialog(title: Text('Title'));
+                    },
+                  );
+                },
+                child: const Text('Show Dialog'),
+              );
+            },
+          ),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(buildFrame(UniqueKey()));
+
+    // Open the dialog.
+    await tester.tap(find.byType(RaisedButton));
+    await tester.pumpAndSettle();
+
+    // Force the Builder to be recreated (new key) which causes outerContext to
+    // be deactivated. If showDialog()'s implementation were to refer to
+    // outerContext again, it would crash.
+    await tester.pumpWidget(buildFrame(UniqueKey()));
+    await tester.pump();
+  });
 }