Merge pull request #1971 from pylaligand/operator

Added a few operators to the Velocity class.

packages/flutter/lib/src/gestures/velocity_tracker.dart

diff --git a/packages/flutter/lib/src/gestures/velocity_tracker.dart b/packages/flutter/lib/src/gestures/velocity_tracker.dart
index 27fe9b4..fe2ca13 100644
--- a/packages/flutter/lib/src/gestures/velocity_tracker.dart
+++ b/packages/flutter/lib/src/gestures/velocity_tracker.dart
@@ -218,6 +218,16 @@
   /// The number of pixels per second of velocity in the x and y directions.
   final Offset pixelsPerSecond;
 
+  Velocity operator -() => new Velocity(pixelsPerSecond: -pixelsPerSecond);
+  Velocity operator -(Velocity other) {
+    return new Velocity(
+        pixelsPerSecond: pixelsPerSecond - other.pixelsPerSecond);
+  }
+  Velocity operator +(Velocity other) {
+    return new Velocity(
+        pixelsPerSecond: pixelsPerSecond + other.pixelsPerSecond);
+  }
+
   bool operator ==(dynamic other) {
     if (other is! Velocity)
       return false;