Fix some info-level warnings from Dart analyzer

sky/sdk/example/game/lib/particle_system.dart

 part of sprites;
 
 class _Particle {
   Vector2 pos;
   Vector2 startPos;
 
   double colorPos;
   double deltaColorPos;
 
   double size;
@@ skipping 58 matching lines @@
   int alphaVar;
   int redVar;
   int greenVar;
   int blueVar;
   TransferMode colorTransferMode;
   TransferMode transferMode;
 
   List<_Particle> _particles;
 
   double _emitCounter;
-  double _elapsedTime;
+  // double _elapsedTime;
   int _numEmittedParticles = 0;
 
   math.Random _rand;
 
   ParticleSystem(this.texture,
                  {this.life: 1.5,
                   this.lifeVar: 1.0,
                   this.posVar: Point.origin,
                   this.startSize: 2.5,
                   this.startSizeVar: 0.5,
@@ skipping 20 matching lines @@
                   this.redVar: 0,
                   this.greenVar: 0,
                   this.blueVar: 0,
                   this.colorTransferMode: TransferMode.multiply,
                   this.transferMode: TransferMode.plus,
                   this.numParticlesToEmit: 0,
                   this.autoRemoveOnFinish: true}) {
     _particles = new List<_Particle>();
     _rand = new math.Random();
     _emitCounter = 0.0;
-    _elapsedTime = 0.0;
+    // _elapsedTime = 0.0;
     if (gravity == null) gravity = new Vector2.zero();
     if (colorSequence == null) colorSequence = new ColorSequence.fromStartAndEndColor(new Color(0xffffffff), new Color(0x00ffffff));
   }
 
   void update(double dt) {
 
     // Create new particles
     double rate = 1.0 / emissionRate;
 
     if (_particles.length < maxParticles) {
       _emitCounter += dt;
     }
 
     while(_particles.length < maxParticles
        && _emitCounter > rate
        && (numParticlesToEmit == 0 || _numEmittedParticles < numParticlesToEmit)) {
       // Add a new particle
       _addParticle();
       _emitCounter -= rate;
     }
 
-    _elapsedTime += dt;
+    // _elapsedTime += dt;
 
     // Iterate over all particles
     for (int i = _particles.length -1; i >= 0; i--) {
       _Particle particle = _particles[i];
 
       // Manage life time
       particle.timeToLive -= dt;
       if (particle.timeToLive <= 0) {
         _particles.removeAt(i);
         continue;
@@ skipping 157 matching lines @@
 
 // TODO: Needs bindings to Skia SkRSXform
 class RSTransform {
   double scos;
   double ssin;
   double tx;
   double ty;
 
   RSTransform(this.scos, this.ssin, this.tx, this.ty);
 }