Convert Align to use FractionalOffset instead of two doubles

It was confusing as to whether these were fractions or pixels.

Fixes #1622

examples/widgets/card_collection.dart

@@ -348,8 +348,7 @@ class CardCollectionState extends State<CardCollection> {
     if (_snapToCenter) {
       Widget indicator = new IgnorePointer(
         child: new Align(
-          horizontal: 0.0,
-          vertical: 0.5,
+          alignment: const FractionalOffset(0.0, 0.5),
           child: new Container(
             height: 1.0,
             decoration: new BoxDecoration(backgroundColor: const Color(0x80FFFFFF))

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

@@ -737,3 +737,27 @@ abstract class RenderBoxContainerDefaultsMixin<ChildType extends RenderBox, Pare
     }
   }
 }
+
+/// An offset that's expressed as a fraction of a Size.
+///
+/// FractionalOffset(0.0, 0.0) represents the top left of the Size,
+/// FractionalOffset(1.0, 1.0) represents the bottom right of the Size.
+class FractionalOffset {
+  const FractionalOffset(this.x, this.y);
+  final double x;
+  final double y;
+  bool operator ==(dynamic other) {
+    if (other is! FractionalOffset)
+      return false;
+    final FractionalOffset typedOther = other;
+    return x == typedOther.x &&
+           y == typedOther.y;
+  }
+  int get hashCode {
+    int value = 373;
+    value = 37 * value + x.hashCode;
+    value = 37 * value + y.hashCode;
+    return value;
+  }
+  String toString() => '$runtimeType($x, $y)';
+}

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

@@ -863,30 +863,6 @@ class RenderDecoratedBox extends RenderProxyBox {
   String debugDescribeSettings(String prefix) => '${super.debugDescribeSettings(prefix)}${prefix}decoration:\n${_painter.decoration.toString(prefix + "  ")}\n';
 }
 
-/// An offset that's expressed as a fraction of a Size.
-///
-/// FractionalOffset(0.0, 0.0) represents the top left of the Size,
-/// FractionalOffset(1.0, 1.0) represents the bottom right of the Size.
-class FractionalOffset {
-  const FractionalOffset(this.x, this.y);
-  final double x;
-  final double y;
-  bool operator ==(dynamic other) {
-    if (other is! FractionalOffset)
-      return false;
-    final FractionalOffset typedOther = other;
-    return x == typedOther.x &&
-           y == typedOther.y;
-  }
-  int get hashCode {
-    int value = 373;
-    value = 37 * value + x.hashCode;
-    value = 37 * value + y.hashCode;
-    return value;
-  }
-  String toString() => '$runtimeType($x, $y)';
-}
-
 /// Applies a transformation before painting its child
 class RenderTransform extends RenderProxyBox {
   RenderTransform({

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

@@ -155,35 +155,22 @@ class RenderPositionedBox extends RenderShiftedBox {
 
   RenderPositionedBox({
     RenderBox child,
-    double horizontal: 0.5,
-    double vertical: 0.5,
+    FractionalOffset alignment: const FractionalOffset(0.5, 0.5),
     ShrinkWrap shrinkWrap: ShrinkWrap.none
-  }) : _horizontal = horizontal,
-       _vertical = vertical,
+  }) : _alignment = alignment,
        _shrinkWrap = shrinkWrap,
        super(child) {
-    assert(horizontal != null);
-    assert(vertical != null);
+    assert(alignment != null);
     assert(shrinkWrap != null);
   }
 
-  double _horizontal;
-  double get horizontal => _horizontal;
-  void set horizontal (double value) {
-    assert(value != null);
-    if (_horizontal == value)
-      return;
-    _horizontal = value;
-    markNeedsLayout();
-  }
-
-  double _vertical;
-  double get vertical => _vertical;
-  void set vertical (double value) {
-    assert(value != null);
-    if (_vertical == value)
+  FractionalOffset get alignment => _alignment;
+  FractionalOffset _alignment;
+  void set alignment (FractionalOffset newAlignment) {
+    assert(newAlignment == null || (newAlignment.x != null && newAlignment.y != null));
+    if (_alignment == newAlignment)
       return;
-    _vertical = value;
+    _alignment = newAlignment;
     markNeedsLayout();
   }
 
@@ -208,14 +195,14 @@ class RenderPositionedBox extends RenderShiftedBox {
                                             _shinkWrapHeight ? child.size.height : double.INFINITY));
       Offset delta = size - child.size;
       final BoxParentData childParentData = child.parentData;
-      childParentData.position = (delta.scale(horizontal, vertical)).toPoint();
+      childParentData.position = (delta.scale(_alignment.x, _alignment.y)).toPoint();
     } else {
       size = constraints.constrain(new Size(_shinkWrapWidth ? 0.0 : double.INFINITY,
                                             _shinkWrapHeight ? 0.0 : double.INFINITY));
     }
   }
 
-  String debugDescribeSettings(String prefix) => '${super.debugDescribeSettings(prefix)}${prefix}horizontal: $horizontal\n${prefix}vertical: $vertical\n';
+  String debugDescribeSettings(String prefix) => '${super.debugDescribeSettings(prefix)}${prefix}alignment: $alignment\n';
 }
 
 class RenderBaseline extends RenderShiftedBox {

packages/flutter/lib/src/widgets/basic.dart

@@ -237,25 +237,20 @@ class Padding extends OneChildRenderObjectWidget {
 class Align extends OneChildRenderObjectWidget {
   Align({
     Key key,
-    this.horizontal: 0.5,
-    this.vertical: 0.5,
+    this.alignment: const FractionalOffset(0.5, 0.5),
     this.shrinkWrap: ShrinkWrap.none,
     Widget child
   }) : super(key: key, child: child) {
-    assert(horizontal != null);
-    assert(vertical != null);
     assert(shrinkWrap != null);
   }
 
-  final double horizontal;
-  final double vertical;
+  final FractionalOffset alignment;
   final ShrinkWrap shrinkWrap;
 
-  RenderPositionedBox createRenderObject() => new RenderPositionedBox(horizontal: horizontal, vertical: vertical, shrinkWrap: shrinkWrap);
+  RenderPositionedBox createRenderObject() => new RenderPositionedBox(alignment: alignment, shrinkWrap: shrinkWrap);
 
   void updateRenderObject(RenderPositionedBox renderObject, Align oldWidget) {
-    renderObject.horizontal = horizontal;
-    renderObject.vertical = vertical;
+    renderObject.alignment = alignment;
     renderObject.shrinkWrap = shrinkWrap;
   }
 }

packages/unit/test/widget/align_test.dart

@@ -9,16 +9,14 @@ void main() {
       tester.pumpWidget(
         new Align(
           child: new Container(),
-          horizontal: 0.75,
-          vertical: 0.75
+          alignment: const FractionalOffset(0.75, 0.75)
         )
       );
 
       tester.pumpWidget(
         new Align(
           child: new Container(),
-          horizontal: 0.5,
-          vertical: 0.5
+          alignment: const FractionalOffset(0.5, 0.5)
         )
       );
     });