Be more explicit about what's state in dismissable

Move _activeCardDragEndPoint near build() so it's more obvious that it
is part of the build state.

Make a couple of functions use setState() since they modify variables
that are used by build().

Add a more detailed comment to the empty setState() call, since those
are dubious in general and need explaining when they occur, lest people
start using them as magic incantations to Make Things Work.

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

@@ -75,14 +75,6 @@ class Dismissable extends StatefulComponent {
       _startResizePerformance();
   }
 
-  Point get _activeCardDragEndPoint {
-    if (!_isActive)
-      return Point.origin;
-    assert(_size != null);
-    double extent = _directionIsYAxis ? _size.height : _size.width;
-    return new Point(_dragExtent.sign * extent * _kDismissCardThreshold, 0.0);
-  }
-
   bool get _isActive {
     return _size != null && (_dragUnderway || _fadePerformance.isAnimating);
   }
@@ -120,9 +112,11 @@ class Dismissable extends StatefulComponent {
   void _handleDragStart() {
     if (_fadePerformance.isAnimating)
       return;
-    _dragUnderway = true;
-    _dragExtent = 0.0;
-    _fadePerformance.progress = 0.0;
+    setState(() {
+      _dragUnderway = true;
+      _dragExtent = 0.0;
+      _fadePerformance.progress = 0.0;
+    });
   }
 
   void _handleDragUpdate(double scrollOffset) {
@@ -149,8 +143,14 @@ class Dismissable extends StatefulComponent {
         break;
     }
 
-    if (oldDragExtent.sign != _dragExtent.sign)
-      setState(() {}); // Rebuild to update the new drag endpoint.
+    if (oldDragExtent.sign != _dragExtent.sign) {
+      setState(() {
+        // Rebuild to update the new drag endpoint.
+        // The sign of _dragExtent is part of our build state;
+        // the actual value is not, it's just used to configure
+        // the performances.
+      });
+    }
     if (!_fadePerformance.isAnimating)
       _fadePerformance.progress = _dragExtent.abs() / (_size.width * _kDismissCardThreshold);
   }
@@ -188,16 +188,18 @@ class Dismissable extends StatefulComponent {
     if (!_isActive || _fadePerformance.isAnimating)
       return;
 
-    _dragUnderway = false;
-    if (_fadePerformance.isCompleted) {
-      _startResizePerformance();
-    } else if (_isFlingGesture(velocity)) {
-      double flingVelocity = _directionIsYAxis ? velocity.dy : velocity.dx;
-      _dragExtent = flingVelocity.sign;
-      _fadePerformance.fling(velocity: flingVelocity.abs() * _kFlingVelocityScale);
-    } else {
-      _fadePerformance.reverse();
-    }
+    setState(() {
+      _dragUnderway = false;
+      if (_fadePerformance.isCompleted) {
+        _startResizePerformance();
+      } else if (_isFlingGesture(velocity)) {
+        double flingVelocity = _directionIsYAxis ? velocity.dy : velocity.dx;
+        _dragExtent = flingVelocity.sign;
+        _fadePerformance.fling(velocity: flingVelocity.abs() * _kFlingVelocityScale);
+      } else {
+        _fadePerformance.reverse();
+      }
+    });
   }
 
   void _handleSizeChanged(Size newSize) {
@@ -206,6 +208,14 @@ class Dismissable extends StatefulComponent {
     });
   }
 
+  Point get _activeCardDragEndPoint {
+    if (!_isActive)
+      return Point.origin;
+    assert(_size != null);
+    double extent = _directionIsYAxis ? _size.height : _size.width;
+    return new Point(_dragExtent.sign * extent * _kDismissCardThreshold, 0.0);
+  }
+
   Widget build() {
     if (_resizePerformance != null) {
       AnimatedValue<double> squashAxisExtent = new AnimatedValue<double>(