Custom context menu example (#1463)

* flutter create

* Copied in from 'contextual-menu-examples' repo

* Works again with the latest updates to the branch

* Clean up anywhere example

* Clean up other examples

* Add to CI

* Updated with release version of context menus, and added platform switcher

* Generated files

* Home icon on your original platform

* Remove web_dashboard from ci, not sure why that change was there...

* Add context_menu to beta channel, but commented out, for the future

* +x permissions on the master script, which I may have accidentally changed before?

* Actual bash comment

* dart format

* Natural default platform'

* A working test, for the email page

* Import order fix

* Test some pages

* More tests

* dart format

experimental/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>((TextSelectionPoint point) {
+    return TextSelectionPoint(
+      box.localToGlobal(point.point),
+      point.direction,
+    );
+  }).toList();
+}