ImageFit.cover doesn't respect alignment

We need to apply the alignment to the source rect because the ouput rect fills
the destination rect.

Fixes #1514

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

@@ -492,8 +492,7 @@ void paintImage({
   ImageFit fit,
   ImageRepeat repeat: ImageRepeat.noRepeat,
   Rect centerSlice,
-  double alignX,
-  double alignY
+  FractionalOffset alignment
 }) {
   assert(canvas != null);
   assert(image != null);
@@ -508,6 +507,7 @@ void paintImage({
     outputSize -= sliceBorder;
     inputSize -= sliceBorder;
   }
+  Point sourcePosition = Point.origin;
   Size sourceSize;
   Size destinationSize;
   fit ??= centerSlice == null ? ImageFit.scaleDown : ImageFit.fill;
@@ -525,10 +525,13 @@ void paintImage({
         destinationSize = new Size(outputSize.width, sourceSize.height * outputSize.width / sourceSize.width);
       break;
     case ImageFit.cover:
-      if (outputSize.width / outputSize.height > inputSize.width / inputSize.height)
+      if (outputSize.width / outputSize.height > inputSize.width / inputSize.height) {
         sourceSize = new Size(inputSize.width, inputSize.width * outputSize.height / outputSize.width);
-      else
+        sourcePosition = new Point(0.0, (inputSize.height - sourceSize.height) * (alignment?.dy ?? 0.5));
+      } else {
         sourceSize = new Size(inputSize.height * outputSize.width / outputSize.height, inputSize.height);
+        sourcePosition = new Point((inputSize.width - sourceSize.width) * (alignment?.dx ?? 0.5), 0.0);
+      }
       destinationSize = outputSize;
       break;
     case ImageFit.none:
@@ -566,8 +569,8 @@ void paintImage({
     // to nearest-neighbor.
     paint.filterQuality = FilterQuality.low;
   }
-  double dx = (outputSize.width - destinationSize.width) * (alignX ?? 0.5);
-  double dy = (outputSize.height - destinationSize.height) * (alignY ?? 0.5);
+  double dx = (outputSize.width - destinationSize.width) * (alignment?.dx ?? 0.5);
+  double dy = (outputSize.height - destinationSize.height) * (alignment?.dy ?? 0.5);
   Point destinationPosition = rect.topLeft + new Offset(dx, dy);
   Rect destinationRect = destinationPosition & destinationSize;
   if (repeat != ImageRepeat.noRepeat) {
@@ -575,7 +578,7 @@ void paintImage({
     canvas.clipRect(rect);
   }
   if (centerSlice == null) {
-    Rect sourceRect = Point.origin & sourceSize;
+    Rect sourceRect = sourcePosition & sourceSize;
     for (Rect tileRect in _generateImageTileRects(rect, destinationRect, repeat))
       canvas.drawImageRect(image, sourceRect, tileRect, paint);
   } else {
@@ -1035,8 +1038,7 @@ class _BoxDecorationPainter extends BoxPainter {
       rect: rect,
       image: image,
       colorFilter: backgroundImage.colorFilter,
-      alignX: backgroundImage.alignment?.dx,
-      alignY: backgroundImage.alignment?.dy,
+      alignment: backgroundImage.alignment,
       fit:  backgroundImage.fit,
       repeat: backgroundImage.repeat
     );

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

@@ -242,8 +242,7 @@ class RenderImage extends RenderBox {
       image: _image,
       colorFilter: _colorFilter,
       fit: _fit,
-      alignX: _alignment?.dx,
-      alignY: _alignment?.dy,
+      alignment: _alignment,
       centerSlice: _centerSlice,
       repeat: _repeat
     );

packages/flutter/test/painting/paint_image_test.dart

@@ -0,0 +1,49 @@
+// 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 'dart:ui' as ui;
+
+import 'package:flutter/painting.dart';
+
+import 'package:test/test.dart';
+
+class TestImage implements ui.Image {
+  TestImage({ this.width, this.height });
+
+  final int width;
+  final int height;
+
+  void dispose() { }
+}
+
+class TestCanvas implements Canvas {
+  final List<Invocation> invocations = <Invocation>[];
+
+  void noSuchMethod(Invocation invocation) {
+    invocations.add(invocation);
+  }
+}
+
+void main() {
+  test("Cover and align", () {
+    TestImage image = new TestImage(width: 300, height: 300);
+    TestCanvas canvas = new TestCanvas();
+    paintImage(
+      canvas: canvas,
+      rect: new Rect.fromLTWH(50.0, 75.0, 200.0, 100.0),
+      image: image,
+      fit: ImageFit.cover,
+      alignment: new FractionalOffset(0.0, 0.5)
+    );
+
+    Invocation command = canvas.invocations.firstWhere((Invocation invocation) {
+      return invocation.memberName == #drawImageRect;
+    });
+
+    expect(command, isNotNull);
+    expect(command.positionalArguments[0], equals(image));
+    expect(command.positionalArguments[1], equals(new Rect.fromLTWH(0.0, 75.0, 300.0, 150.0)));
+    expect(command.positionalArguments[2], equals(new Rect.fromLTWH(50.0, 75.0, 200.0, 100.0)));
+  });
+}