[Material] Fix showDialog crasher caused by old contexts

[johnsonmh]

Description

• The showDialog function takes a context parameter. That parameter is used to get the current Theme.of(context) when showDialog is called. The showDialog dart docs say that after showDialog is called, its widget can safely be removed from the tree, since context is only used when the method is called. This was not actually true, since the Theme.of(context) call was happening inside the pageBuilder, even though the context being used was from the original function call. This meant that it was possible for Theme.of(context) to be called with an already deactivated build context, throwing an error.

Related Issues

• fixes ShowDialog is using the wrong context #28505

Tests

I added the following tests:

// 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: Material(
        child: 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'),
              );
            },
          ),
        ),
      ),
    );
  }

  // First time build.
  await tester.pumpWidget(buildFrame(UniqueKey()));

  // Open the dialog.
  await tester.tap(find.byType(RaisedButton));
  await tester.pumpAndSettle();

  // Second time build.
  await tester.pumpWidget(buildFrame(UniqueKey()));

  // App crashes when we try to open the dialog, because it tries to get the
  // Theme from the old context.
  await tester.tap(find.byType(RaisedButton));
});

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Breaking Change

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• Yes, this is a breaking change (Please read Handling breaking changes). Replace this with a link to the e-mail where you asked for input on this proposed change.
• No, this is not a breaking change.

[rami-a]

LGTM with one small comment

[HansMuller]

LGTM