[Gallery] Fix directory structure (#312)

gallery/lib/studies/crane/border_tab_indicator.dart

@@ -0,0 +1,52 @@
+// Copyright 2019 The Flutter team. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter/widgets.dart';
+
+class BorderTabIndicator extends Decoration {
+  BorderTabIndicator({this.indicatorHeight, this.textScaleFactor}) : super();
+
+  final double indicatorHeight;
+  final double textScaleFactor;
+
+  @override
+  _BorderPainter createBoxPainter([VoidCallback onChanged]) {
+    return _BorderPainter(
+        this, indicatorHeight, this.textScaleFactor, onChanged);
+  }
+}
+
+class _BorderPainter extends BoxPainter {
+  _BorderPainter(
+    this.decoration,
+    this.indicatorHeight,
+    this.textScaleFactor,
+    VoidCallback onChanged,
+  )   : assert(decoration != null),
+        assert(indicatorHeight >= 0),
+        super(onChanged);
+
+  final BorderTabIndicator decoration;
+  final double indicatorHeight;
+  final double textScaleFactor;
+
+  @override
+  void paint(Canvas canvas, Offset offset, ImageConfiguration configuration) {
+    assert(configuration != null);
+    assert(configuration.size != null);
+    final horizontalInset = 16 - 4 * textScaleFactor;
+    final rect = Offset(offset.dx + horizontalInset,
+            (configuration.size.height / 2) - indicatorHeight / 2 - 1) &
+        Size(configuration.size.width - 2 * horizontalInset, indicatorHeight);
+    final paint = Paint();
+    paint.color = Colors.white;
+    paint.style = PaintingStyle.stroke;
+    paint.strokeWidth = 2;
+    canvas.drawRRect(
+      RRect.fromRectAndRadius(rect, Radius.circular(56)),
+      paint,
+    );
+  }
+}