Specs: strip @nonnull annotations since they're too verbose

sky/specs/pointer.md

 Pointer Events
 ==============
 
 Scope
 -----
 
 The following input devices are supported by sky:
  - fingers on multitouch screens
  - mice, including mouse wheels
  - styluses on screens
@@ skipping 348 matching lines @@
                  this.pressure, this.minPressure, this.maxPressure,
                  this.distance, this.minDistance, this.maxDistance,
                  this.radiusMajor, this.radiusMinor, this.minRadius, this.maxRadius,
                  this.orientation, this.tilt
                }): super();
 
   final int pointer;
   final PointerKind kind;
   final double x; // logical pixels
   final double y; // logical pixels
-  final @nonnull double dx; // logical pixels
-  final @nonnull double dy; // logical pixels
+  final double dx; // logical pixels
+  final double dy; // logical pixels
 
-  final @nonnull int buttons; // bit field
+  final int buttons; // bit field
   static const int primaryMouseButton = 0x01;
   static const int secondaryMouseButton = 0x02;
   static const int primaryStylusButton = 0x02;
   static const int middleMouseButton = 0x04;
   static const int secondaryStylusButton = 0x04;
   static const int backButton = 0x08;
   static const int forwardButton = 0x10;
 
-  final @nonnull bool down;
-  final @nonnull bool primary;
-  final @nonnull bool obscured;
+  final bool down;
+  final bool primary;
+  final bool obscured;
 
   // if down != true, these are all null
   final double pressure; // normalised, 0.0 means none, 1.0 means "normal"
   final double minPressure; // 0 <= minPressure <= 1.0
   final double maxPressure; // maxPressure >= 1.0
 
   // if kind != touch, stylus, or invertedStylus, these are all null
   final double distance; // logical pixels
   final double minDistance; // logical pixels
   final double maxDistance; // logical pixels
@@ skipping 78 matching lines @@
 ```dart
 class WheelEvent extends Event {
   PointerEvent({ bool bubbles,
                  this.wheel,
                  this.delta: 0.0,
                  this.pointer,
                  this.x, this.y,
                }): super(bubbles: bubbles);
 
   final int wheel;
-  final @nonnull double delta; // revolutions (or fractions thereof)
+  final double delta; // revolutions (or fractions thereof)
   final int pointer;
   final double x; // logical pixels
   final double y; // logical pixels
 
   bool get bubbles => false;
 }
 ```