Add SizedBox.fromSize() constructor (#8144)

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

@@ -820,15 +820,24 @@ class CustomMultiChildLayout extends MultiChildRenderObjectWidget {
 /// sizes itself to fit the parent. It is equivalent to setting [width] and
 /// [height] to [double.INFINITY].
 class SizedBox extends SingleChildRenderObjectWidget {
-  /// Creates a box of a specific size.
+  /// Creates a fixed size box. The [width] and [height] parameters can be null
+  /// to indicate that the size of the box should not be constrained in
+  /// the corresponding dimension.
   const SizedBox({ Key key, this.width, this.height, Widget child })
     : super(key: key, child: child);
 
+  /// Creates a box that will become as large as its parent allows.
   const SizedBox.expand({ Key key, Widget child })
     : width = double.INFINITY,
       height = double.INFINITY,
       super(key: key, child: child);
 
+  /// Creates a box with the specified size.
+  SizedBox.fromSize({ Key key, Widget child, Size size })
+    : width = size?.width,
+      height = size?.height,
+      super(key: key, child: child);
+
   /// If non-null, requires the child to have exactly this width.
   final double width;
 

packages/flutter/test/widgets/sized_box_test.dart

@@ -6,6 +6,32 @@ import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 void main() {
+  testWidgets('SizedBox constructors', (WidgetTester tester) async {
+    final SizedBox a = const SizedBox();
+    expect(a.width, isNull);
+    expect(a.height, isNull);
+
+    final SizedBox b = const SizedBox(width: 10.0);
+    expect(b.width, 10.0);
+    expect(b.height, isNull);
+
+    final SizedBox c = const SizedBox(width: 10.0, height: 20.0);
+    expect(c.width, 10.0);
+    expect(c.height, 20.0);
+
+    final SizedBox d = new SizedBox.fromSize();
+    expect(d.width, isNull);
+    expect(d.height, isNull);
+
+    final SizedBox e = new SizedBox.fromSize(size: const Size(1.0, 2.0));
+    expect(e.width, 1.0);
+    expect(e.height, 2.0);
+
+    final SizedBox f = const SizedBox.expand();
+    expect(f.width, double.INFINITY);
+    expect(f.height, double.INFINITY);
+  });
+
   testWidgets('SizedBox - no child', (WidgetTester tester) async {
     GlobalKey patient = new GlobalKey();