Update clip geometry during hitTest if needed (#6091)

The code previously assumed that paint would be called before hitTest, but that
assumption isn't valid because RenderOpacity hit tests its children without
painting them.

Fixes #6089

packages/flutter/lib/src/rendering/box.dart

@@ -1269,6 +1269,13 @@ abstract class RenderBox extends RenderObject {
   /// The caller is responsible for transforming [position] into the local
   /// coordinate space of the callee.  The callee is responsible for checking
   /// whether the given position is within its bounds.
+  ///
+  /// Hit testing requires layout to be up-to-date but does not require painting
+  /// to be up-to-date. That means a render object can rely upon [performLayout]
+  /// having been called in [hitTest] but cannot rely upon [paint] having been
+  /// called. For example, a render object might be a child of a [RenderOpacity]
+  /// object, which calls [hitTest] on its children when its opacity is zero
+  /// even through it does not [paint] its children.
   bool hitTest(HitTestResult result, { @required Point position }) {
     assert(() {
       if (needsLayout) {

packages/flutter/lib/src/rendering/proxy_box.dart

@@ -941,6 +941,7 @@ class RenderClipRect extends _RenderCustomClip<Rect> {
   @override
   bool hitTest(HitTestResult result, { Point position }) {
     if (_clipper != null) {
+      _updateClip();
       assert(_clip != null);
       if (!_clip.contains(position))
         return false;
@@ -1033,6 +1034,7 @@ class RenderClipOval extends _RenderCustomClip<Rect> {
 
   @override
   bool hitTest(HitTestResult result, { Point position }) {
+    _updateClip();
     assert(_clip != null);
     Point center = _clip.center;
     // convert the position to an offset from the center of the unit circle
@@ -1082,6 +1084,7 @@ class RenderClipPath extends _RenderCustomClip<Path> {
   @override
   bool hitTest(HitTestResult result, { Point position }) {
     if (_clipper != null) {
+      _updateClip();
       assert(_clip != null);
       if (!_clip.contains(position))
         return false;

packages/flutter/test/widget/clip_test.dart

@@ -78,6 +78,29 @@ void main() {
     log.clear();
   });
 
+  testWidgets('Transparent ClipOval hit test', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new Opacity(
+        opacity: 0.0,
+        child: new ClipOval(
+          child: new GestureDetector(
+            behavior: HitTestBehavior.opaque,
+            onTap: () { log.add('tap'); },
+          )
+        )
+      )
+    );
+    expect(log, equals(<String>[]));
+
+    await tester.tapAt(new Point(10.0, 10.0));
+    expect(log, equals(<String>[]));
+    log.clear();
+
+    await tester.tapAt(new Point(400.0, 300.0));
+    expect(log, equals(<String>['tap']));
+    log.clear();
+  });
+
   testWidgets('ClipRect', (WidgetTester tester) async {
     await tester.pumpWidget(
       new Align(