Fix some info-level warnings from Dart analyzer

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

Index: sky/sdk/example/game/lib/particle_system.dart
diff --git a/sky/sdk/example/game/lib/particle_system.dart b/sky/sdk/example/game/lib/particle_system.dart
index a64210bd12528deb121f75475e1613801f714a4b..c9a50fd715c3ba9986ac911dae598085312fb51d 100644
--- a/sky/sdk/example/game/lib/particle_system.dart
+++ b/sky/sdk/example/game/lib/particle_system.dart
@@ -76,7 +76,7 @@ class ParticleSystem extends Node {
   List<_Particle> _particles;
 
   double _emitCounter;
-  double _elapsedTime;
+  // double _elapsedTime;
   int _numEmittedParticles = 0;
 
   math.Random _rand;
@@ -117,7 +117,7 @@ class ParticleSystem extends Node {
     _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));
   }
@@ -139,7 +139,7 @@ class ParticleSystem extends Node {
       _emitCounter -= rate;
     }
 
-    _elapsedTime += dt;
+    // _elapsedTime += dt;
 
     // Iterate over all particles
     for (int i = _particles.length -1; i >= 0; i--) {