Spacer should only flex in the main axis direction. (#18052)

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

@@ -59,8 +59,9 @@ class Spacer extends StatelessWidget {
   Widget build(BuildContext context) {
     return new Expanded(
       flex: flex,
-      child: new ConstrainedBox(
-        constraints: const BoxConstraints.expand(),
+      child: const SizedBox(
+        height: 0.0,
+        width: 0.0,
       ),
     );
   }

packages/flutter/test/widgets/spacer_test.dart

@@ -15,8 +15,8 @@ void main() {
       ],
     ));
     final Rect spacerRect = tester.getRect(find.byType(Spacer));
-    expect(spacerRect.size, const Size(800.0, 580.0));
-    expect(spacerRect.topLeft, const Offset(0.0, 10.0));
+    expect(spacerRect.size, const Size(0.0, 580.0));
+    expect(spacerRect.topLeft, const Offset(400.0, 10.0));
   });
 
   testWidgets('Spacer takes up space proportional to flex.', (WidgetTester tester) async {
@@ -42,7 +42,7 @@ void main() {
     final Rect spacer2Rect = tester.getRect(find.byType(Spacer).at(1));
     final Rect spacer3Rect = tester.getRect(find.byType(Spacer).at(2));
     final Rect spacer4Rect = tester.getRect(find.byType(Spacer).at(3));
-    expect(spacer1Rect.size.height, 600.0);
+    expect(spacer1Rect.size.height, 0.0);
     expect(spacer1Rect.size.width, closeTo(93.8, 0.1));
     expect(spacer1Rect.left, closeTo(696.3, 0.1));
     expect(spacer2Rect.size.width, closeTo(93.8, 0.1));
@@ -52,4 +52,22 @@ void main() {
     expect(spacer4Rect.size.width, spacer3Rect.size.width * 2.0);
     expect(spacer4Rect.left, closeTo(10.0, 0.1));
   });
+  
+  testWidgets('Spacer takes up space.', (WidgetTester tester) async {
+    await tester.pumpWidget(new UnconstrainedBox(
+      constrainedAxis: Axis.vertical,
+      child: new Column(
+        children: const <Widget>[
+          const SizedBox(width: 20.0, height: 10.0),
+          const Spacer(),
+          const SizedBox(width: 10.0, height: 10.0),
+        ],
+      ),
+    ));
+    final Rect spacerRect = tester.getRect(find.byType(Spacer));
+    final Rect flexRect = tester.getRect(find.byType(Column));
+    expect(spacerRect.size, const Size(0.0, 580.0));
+    expect(spacerRect.topLeft, const Offset(400.0, 10.0));
+    expect(flexRect, new Rect.fromLTWH(390.0, 0.0, 20.0, 600.0));
+  });
 }