Drawer edge swipe convers entire screen

When I tightened up the layout constraints for the Drawer in the
Scaffold, I ended up making the edge swipe detector cover the entire
screen.  This patch fixes that issue by putting the gesture detector for
the edge swipe just around the container with the proper width. We now
use a global key to maintain the state across hierarchy changes.

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

@@ -152,20 +152,27 @@ class DrawerControllerState extends State<DrawerController> {
   }
 
   final AnimatedColorValue _color = new AnimatedColorValue(Colors.transparent, end: Colors.black54);
+  final GlobalKey _gestureDetectorKey = new GlobalKey();
 
   Widget build(BuildContext context) {
-    HitTestBehavior behavior;
-    Widget child;
     if (_performance.status == PerformanceStatus.dismissed) {
-      behavior = HitTestBehavior.translucent;
-      child = new Align(
+      return new Align(
         alignment: const FractionalOffset(0.0, 0.5),
-        widthFactor: 1.0,
+        child: new GestureDetector(
+          key: _gestureDetectorKey,
+          onHorizontalDragUpdate: _move,
+          onHorizontalDragEnd: _settle,
+          behavior: HitTestBehavior.translucent,
           child: new Container(width: _kEdgeDragWidth)
+        )
       );
     } else {
       _performance.updateVariable(_color);
-      child = new RepaintBoundary(
+      return new GestureDetector(
+        key: _gestureDetectorKey,
+        onHorizontalDragUpdate: _move,
+        onHorizontalDragEnd: _settle,
+        child: new RepaintBoundary(
           child: new Stack(<Widget>[
             new GestureDetector(
               onTap: close,
@@ -196,13 +203,8 @@ class DrawerControllerState extends State<DrawerController> {
               )
             )
           ])
+        )
       );
     }
-    return new GestureDetector(
-      onHorizontalDragUpdate: _move,
-      onHorizontalDragEnd: _settle,
-      behavior: behavior,
-      child: child
-    );
   }
 }