Fix a typo in Dialog

If action: was specified, content: was added twice.

R=jackson@google.com

Review URL: https://codereview.chromium.org/1188303002.

sdk/lib/widgets/dialog.dart

@@ -33,7 +33,7 @@ class Dialog extends Component {
       children.add(content);
 
     if (actions != null)
-      children.add(content);
+      children.add(actions);
 
     return new Stack([
       new Listener(

tests/widgets/dialog-expected.txt

@@ -13,12 +13,14 @@ PAINT FOR FRAME #2 ----------------------------------------------
 2 |  |  | drawRect(Rect.fromLTRB(0.0, 0.0, 800.0, 600.0), Paint(Color(0x7f000000)))
 2 |  | paintChild RenderPositionedBox at Point(0.0, 0.0)
 2 |  |  | TestDisplayList() constructor: 800.0 x 600.0
-2 |  |  | paintChild RenderConstrainedBox at Point(260.0, 284.0)
+2 |  |  | paintChild RenderConstrainedBox at Point(260.0, 276.0)
 2 |  |  |  | TestDisplayList() constructor: 800.0 x 600.0
 2 |  |  |  | drawRRect(Instance of 'RRect', Paint(Color(0xfffafafa)))
 2 |  |  |  | paintChild RenderParagraph at Point(0.0, 0.0)
 2 |  |  |  |  | TestDisplayList() constructor: 800.0 x 600.0
 2 |  |  |  | paintChild RenderParagraph at Point(0.0, 16.0)
 2 |  |  |  |  | TestDisplayList() constructor: 800.0 x 600.0
+2 |  |  |  | paintChild RenderParagraph at Point(0.0, 32.0)
+2 |  |  |  |  | TestDisplayList() constructor: 800.0 x 600.0
 ------------------------------------------------------------------------
 PAINTED 2 FRAMES

tests/widgets/dialog.dart

@@ -13,7 +13,8 @@ main() async {
   await tester.test(() {
     return new Dialog(
       title: new Text("I am a title"),
-      content: new Text("I am some content")
+      content: new Text("I am some content"),
+      actions: new Text("I am some actions")
     );
   });