修复Vector2.zero引起的引用混乱问题

source/src/Math/MathHelper.ts

@@ -50,9 +50,16 @@ module es {
             return value;
         }
 
+        /**
+         * 给定圆心、半径和角度，得到圆周上的一个点。0度是3点钟。
+         * @param circleCenter
+         * @param radius
+         * @param angleInDegrees
+         */
         public static pointOnCirlce(circleCenter: Vector2, radius: number, angleInDegrees: number) {
             let radians = MathHelper.toRadians(angleInDegrees);
-            return new Vector2(Math.cos(radians) * radians + circleCenter.x, Math.sin(radians) * radians + circleCenter.y);
+            return new Vector2(Math.cos(radians) * radians + circleCenter.x,
+                Math.sin(radians) * radians + circleCenter.y);
         }
 
         /**

source/src/Math/Vector2.ts

@@ -1,10 +1,6 @@
 module es {
     /** 2d 向量 */
     export class Vector2 {
-        private static readonly unitYVector = new Vector2(0, 1);
-        private static readonly unitXVector = new Vector2(1, 0);
-        private static readonly unitVector2 = new Vector2(1, 1);
-        private static readonly zeroVector2 = new Vector2(0, 0);
         public x: number = 0;
         public y: number = 0;
 
@@ -19,19 +15,19 @@ module es {
         }
 
         public static get zero() {
-            return Vector2.zeroVector2;
+            return new Vector2(0, 0);
         }
 
         public static get one() {
-            return Vector2.unitVector2;
+            return new Vector2(1, 1);
         }
 
         public static get unitX() {
-            return Vector2.unitXVector;
+            return new Vector2(1, 0);
         }
 
         public static get unitY() {
-            return Vector2.unitYVector;
+            return new Vector2(0, 1);
         }
 
         /**