iOS Dialog blur, brightness, and layout (#18381)

Rewrote CupertinoAlertDialog to look nearly identical to an alert dialog in iOS. This includes considerations for blur, translucent white background color, button sizing, gap dividers between buttons, and text scaling layout behavior. (#18381)

packages/flutter/lib/src/painting/box_decoration.dart

@@ -83,11 +83,11 @@ class BoxDecoration extends Decoration {
     this.backgroundBlendMode,
     this.shape = BoxShape.rectangle,
   }) : assert(shape != null),
-       // TODO(mattcarroll): Use "backgroundBlendMode == null" when Dart #31140 is in.
+       // TODO(mattcarroll): Use "backgroundBlendMode == null" when https://github.com/dart-lang/sdk/issues/31140 is in.
        assert(
-       identical(backgroundBlendMode, null) || color != null || gradient != null,
-       'backgroundBlendMode applies to BoxDecoration\'s background color or'
-           'gradient, but no color or gradient were provided.'
+         identical(backgroundBlendMode, null) || color != null || gradient != null,
+         'backgroundBlendMode applies to BoxDecoration\'s background color or '
+         'gradient, but no color or gradient were provided.'
        );
 
   @override
@@ -146,10 +146,10 @@ class BoxDecoration extends Decoration {
 
   /// The blend mode applied to the [color] or [gradient] background of the box.
   ///
-  /// If no [backgroundBlendMode] is provided, then the default painting blend
+  /// If no [backgroundBlendMode] is provided then the default painting blend
   /// mode is used.
   ///
-  /// If no [color] or [gradient] is provided, then blend mode has no impact.
+  /// If no [color] or [gradient] is provided then the blend mode has no impact.
   final BlendMode backgroundBlendMode;
 
   /// The shape to fill the background [color], [gradient], and [image] into and

packages/flutter_test/lib/src/controller.dart

@@ -273,6 +273,17 @@ abstract class WidgetController {
     });
   }
 
+  /// Dispatch a pointer down at the center of the given widget, assuming it is
+  /// exposed.
+  ///
+  /// If the center of the widget is not exposed, this might send events to
+  /// another object.
+  Future<TestGesture> press(Finder finder, { int pointer }) {
+    return TestAsyncUtils.guard<TestGesture>(() {
+      return startGesture(getCenter(finder), pointer: pointer);
+    });
+  }
+
   /// Dispatch a pointer down / pointer up sequence (with a delay of
   /// [kLongPressTimeout] + [kPressTimeout] between the two events) at the
   /// center of the given widget, assuming it is exposed.

packages/flutter_test/lib/src/test_async_utils.dart

@@ -57,7 +57,7 @@ class TestAsyncUtils {
   /// this one before this one has finished will throw an exception.
   ///
   /// This method first calls [guardSync].
-  static Future<Null> guard(Future<Null> body()) {
+  static Future<T> guard<T>(Future<T> body()) {
     guardSync();
     final Zone zone = Zone.current.fork(
       zoneValues: <dynamic, dynamic>{
@@ -66,8 +66,9 @@ class TestAsyncUtils {
     );
     final _AsyncScope scope = new _AsyncScope(StackTrace.current, zone);
     _scopeStack.add(scope);
-    final Future<Null> result = scope.zone.run(body);
-    Future<Null> completionHandler(dynamic error, StackTrace stack) {
+    final Future<T> result = scope.zone.run<Future<T>>(body);
+    T resultValue; // This is set when the body of work completes with a result value.
+    Future<T> completionHandler(dynamic error, StackTrace stack) {
       assert(_scopeStack.isNotEmpty);
       assert(_scopeStack.contains(scope));
       bool leaked = false;
@@ -102,11 +103,12 @@ class TestAsyncUtils {
         throw new FlutterError(message.toString().trimRight());
       }
       if (error != null)
-        return new Future<Null>.error(error, stack);
-      return new Future<Null>.value(null);
+        return new Future<T>.error(error, stack);
+      return new Future<T>.value(resultValue);
     }
-    return result.then<Null>(
-      (Null value) {
+    return result.then<T>(
+      (T value) {
+        resultValue = value;
         return completionHandler(null, null);
       },
       onError: completionHandler