Factor out Border and BorderRadius into their own files. (#12055)

This will make it more tractable to convert them for RTL.

packages/flutter/lib/painting.dart

@@ -18,6 +18,8 @@
 library painting;
 
 export 'src/painting/basic_types.dart';
+export 'src/painting/border.dart';
+export 'src/painting/border_radius.dart';
 export 'src/painting/box_fit.dart';
 export 'src/painting/box_painter.dart';
 export 'src/painting/colors.dart';

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

+// Copyright 2015 The Chromium Authors. 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/foundation.dart';
+
+import 'basic_types.dart';
+
+/// An immutable set of radii for each corner of a rectangle.
+///
+/// Used by [BoxDecoration] when the shape is a [BoxShape.rectangle].
+@immutable
+class BorderRadius {
+  /// Creates a border radius where all radii are [radius].
+  const BorderRadius.all(Radius radius) : this.only(
+    topLeft: radius,
+    topRight: radius,
+    bottomRight: radius,
+    bottomLeft: radius
+  );
+
+  /// Creates a border radius where all radii are [Radius.circular(radius)].
+  BorderRadius.circular(double radius) : this.all(
+    new Radius.circular(radius)
+  );
+
+  /// Creates a vertically symmetric border radius where the top and bottom
+  /// sides of the rectangle have the same radii.
+  const BorderRadius.vertical({
+    Radius top: Radius.zero,
+    Radius bottom: Radius.zero
+  }) : this.only(
+    topLeft: top,
+    topRight: top,
+    bottomRight: bottom,
+    bottomLeft: bottom
+  );
+
+  /// Creates a horizontally symmetrical border radius where the left and right
+  /// sides of the rectangle have the same radii.
+  const BorderRadius.horizontal({
+    Radius left: Radius.zero,
+    Radius right: Radius.zero
+  }) : this.only(
+    topLeft: left,
+    topRight: right,
+    bottomRight: right,
+    bottomLeft: left
+  );
+
+  /// Creates a border radius with only the given non-zero values. The other
+  /// corners will be right angles.
+  const BorderRadius.only({
+    this.topLeft: Radius.zero,
+    this.topRight: Radius.zero,
+    this.bottomRight: Radius.zero,
+    this.bottomLeft: Radius.zero
+  });
+
+  /// A border radius with all zero radii.
+  static const BorderRadius zero = const BorderRadius.all(Radius.zero);
+
+  /// The top-left [Radius].
+  final Radius topLeft;
+  /// The top-right [Radius].
+  final Radius topRight;
+  /// The bottom-right [Radius].
+  final Radius bottomRight;
+  /// The bottom-left [Radius].
+  final Radius bottomLeft;
+
+  /// Linearly interpolates between two [BorderRadius] objects.
+  ///
+  /// If either is null, this function interpolates from [BorderRadius.zero].
+  static BorderRadius lerp(BorderRadius a, BorderRadius b, double t) {
+    if (a == null && b == null)
+      return null;
+    return new BorderRadius.only(
+      topLeft: Radius.lerp(a.topLeft, b.topLeft, t),
+      topRight: Radius.lerp(a.topRight, b.topRight, t),
+      bottomRight: Radius.lerp(a.bottomRight, b.bottomRight, t),
+      bottomLeft: Radius.lerp(a.bottomLeft, b.bottomLeft, t)
+    );
+  }
+
+  /// Creates a [RRect] from the current border radius and a [Rect].
+  RRect toRRect(Rect rect) {
+    return new RRect.fromRectAndCorners(
+      rect,
+      topLeft: topLeft,
+      topRight: topRight,
+      bottomRight: bottomRight,
+      bottomLeft: bottomLeft
+    );
+  }
+
+  @override
+  bool operator ==(dynamic other) {
+    if (identical(this, other))
+      return true;
+    if (runtimeType != other.runtimeType)
+      return false;
+    final BorderRadius typedOther = other;
+    return topLeft == typedOther.topLeft &&
+           topRight == typedOther.topRight &&
+           bottomRight == typedOther.bottomRight &&
+           bottomLeft == typedOther.bottomLeft;
+  }
+
+  @override
+  int get hashCode => hashValues(topLeft, topRight, bottomRight, bottomLeft);
+
+  @override
+  String toString() {
+    if (topLeft == topRight &&
+        topRight == bottomRight &&
+        bottomRight == bottomLeft) {
+      if (topLeft == Radius.zero)
+        return 'BorderRadius.zero';
+      if (topLeft.x == topLeft.y)
+        return 'BorderRadius.circular(${topLeft.x.toStringAsFixed(1)})';
+      return 'BorderRadius.all($topLeft)';
+    }
+    if (topLeft == Radius.zero ||
+        topRight == Radius.zero ||
+        bottomLeft == Radius.zero ||
+        bottomRight == Radius.zero) {
+      final StringBuffer result = new StringBuffer();
+      result.write('BorderRadius.only(');
+      bool comma = false;
+      if (topLeft != Radius.zero) {
+        result.write('topLeft: $topLeft');
+        comma = true;
+      }
+      if (topRight != Radius.zero) {
+        if (comma)
+          result.write(', ');
+        result.write('topRight: $topRight');
+        comma = true;
+      }
+      if (bottomLeft != Radius.zero) {
+        if (comma)
+          result.write(', ');
+        result.write('bottomLeft: $bottomLeft');
+        comma = true;
+      }
+      if (bottomRight != Radius.zero) {
+        if (comma)
+          result.write(', ');
+        result.write('bottomRight: $bottomRight');
+      }
+      result.write(')');
+      return result.toString();
+    }
+    return 'BorderRadius($topLeft, $topRight, $bottomRight, $bottomLeft)';
+  }
+}