Move context menus out of experimental (#2134)

Now that the context menu code is on stable, move the sample out of
experimental.

Fixes https://github.com/flutter/samples/issues/2110

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

---------

Co-authored-by: Brett Morgan <brett.morgan@gmail.com>

context_menus/test/utils.dart

@@ -0,0 +1,46 @@
+import 'package:flutter/cupertino.dart';
+import 'package:flutter/foundation.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+// Returns the first RenderEditable.
+RenderEditable findRenderEditable(WidgetTester tester) {
+  final RenderObject root = tester.renderObject(find.byType(EditableText));
+  expect(root, isNotNull);
+
+  late RenderEditable renderEditable;
+  void recursiveFinder(RenderObject child) {
+    if (child is RenderEditable) {
+      renderEditable = child;
+      return;
+    }
+    child.visitChildren(recursiveFinder);
+  }
+
+  root.visitChildren(recursiveFinder);
+  expect(renderEditable, isNotNull);
+  return renderEditable;
+}
+
+Offset textOffsetToPosition(WidgetTester tester, int offset) {
+  final RenderEditable renderEditable = findRenderEditable(tester);
+  final List<TextSelectionPoint> endpoints = globalize(
+    renderEditable.getEndpointsForSelection(
+      TextSelection.collapsed(offset: offset),
+    ),
+    renderEditable,
+  );
+  expect(endpoints.length, 1);
+  return endpoints[0].point + const Offset(kIsWeb ? 1.0 : 0.0, -2.0);
+}
+
+List<TextSelectionPoint> globalize(
+    Iterable<TextSelectionPoint> points, RenderBox box) {
+  return points.map<TextSelectionPoint>((point) {
+    return TextSelectionPoint(
+      box.localToGlobal(point.point),
+      point.direction,
+    );
+  }).toList();
+}