make some classes static and some fields readonly

Shared/Clock.cs

@@ -1,7 +1,7 @@
 using System;
 
 namespace SemiColinGames {
-  public class Clock {
+  public static class Clock {
     public static void AddModelTime(double seconds) {
       ModelTime += TimeSpan.FromSeconds(seconds);
     }

Shared/FSM.cs

@@ -9,7 +9,7 @@ namespace SemiColinGames {
   }
 
   public class FSM<T> {
-    Dictionary<string, IState<T>> states;
+    readonly Dictionary<string, IState<T>> states;
     IState<T> state;
 
     public FSM(Dictionary<string, IState<T>> states, string initial) {

Shared/Line.cs

@@ -2,7 +2,7 @@
 using System;
 
 namespace SemiColinGames {
-  public class Line {
+  public static class Line {
     public static Point[] Rasterize(Point p1, Point p2) {
       return Line.Rasterize(p1.X, p1.Y, p2.X, p2.Y);
     }

Shared/Timer.cs

@@ -7,8 +7,8 @@ namespace SemiColinGames {
     private readonly Stopwatch stopwatch = new Stopwatch();
     private readonly double targetTime;
     private readonly string name;
+    private readonly int[] histogram = new int[21];
     private double startTime = 0.0;
-    private int[] histogram = new int[21];
     private int totalFrames = 0;
 
     public Timer(double targetTime, string name) {