Fix remaining analyzer warnings in Sky

sky/sdk/lib/animation/generators.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'dart:async';
-import 'dart:math' as math;
 
 import '../base/scheduler.dart' as scheduler;
 import 'curves.dart';
 import 'mechanics.dart';
 
 abstract class Generator {
   Stream<double> get onTick; // TODO(ianh): rename this to tickStream
   void cancel();
 }
 
@@ skipping 136 matching lines @@
   double _update(double timeStamp) {
     double previousTime = _previousTime;
     _previousTime = timeStamp;
     if (previousTime == 0.0)
       return timeStamp;
     double deltaT = timeStamp - previousTime;
     system.update(deltaT);
     return timeStamp;
   }
 }