Added a stretch Positioned constructor. (#5894)

Fixes #5835.

dev/manual_tests/card_collection.dart

@@ -363,11 +363,7 @@ class CardCollectionState extends State<CardCollection> {
     // size of the background,card Stack will be based only on the card. The
     // Viewport ensures that when the card's resize animation occurs, the
     // background (text and icons) will just be clipped, not resized.
-    Widget background = new Positioned(
-      top: 0.0,
-      right: 0.0,
-      bottom: 0.0,
-      left: 0.0,
+    Widget background = new Positioned.stretch(
       child: new Container(
         margin: const EdgeInsets.all(4.0),
         child: new Viewport(

examples/flutter_gallery/lib/demo/cards_demo.dart

@@ -61,11 +61,7 @@ class TravelDestinationItem extends StatelessWidget {
               height: 184.0,
               child: new Stack(
                 children: <Widget>[
-                  new Positioned(
-                    left: 0.0,
-                    top: 0.0,
-                    bottom: 0.0,
-                    right: 0.0,
+                  new Positioned.stretch(
                     child: new Image.asset(
                       destination.assetName,
                       fit: ImageFit.cover

packages/flutter/lib/src/material/grid_tile.dart

@@ -38,11 +38,7 @@ class GridTile extends StatelessWidget {
       return child;
 
     final List<Widget> children = <Widget>[
-      new Positioned(
-        top: 0.0,
-        left: 0.0,
-        bottom: 0.0,
-        right: 0.0,
+      new Positioned.stretch(
         child: child
       )
     ];

packages/flutter/lib/src/material/tooltip.dart

@@ -265,11 +265,7 @@ class _TooltipOverlay extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    return new Positioned(
-      top: 0.0,
-      left: 0.0,
-      right: 0.0,
-      bottom: 0.0,
+    return new Positioned.stretch(
       child: new IgnorePointer(
         child: new CustomSingleChildLayout(
           delegate: new _TooltipPositionDelegate(

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

@@ -1517,6 +1517,19 @@ class Positioned extends ParentDataWidget<Stack> {
        bottom = null,
        super(key: key, child: child);
 
+  /// Creates a Positioned object with [left], [top], [right], and [bottom] set
+  /// to 0.0 unless a value for them is passed.
+  Positioned.stretch({
+    Key key,
+    Widget child,
+    this.left: 0.0,
+    this.top: 0.0,
+    this.right: 0.0,
+    this.bottom: 0.0
+  }) : width = null,
+       height = null,
+       super(key: key, child: child);
+
   /// The distance that the child's left edge is inset from the left of the stack.
   ///
   /// Only two out of the three horizontal values ([left], [right], [width]) can be