Removing very slow precondition checks. Removing these yields 10%

speedup of S2Polygon.isValid().

This particular hot spot was identified using YourKit profiler.
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=25256987

Author: Ilyia Kaushansky

src/com/google/common/geometry/S2.java

@@ -521,6 +521,9 @@ public static boolean simpleCCW(S2Point a, S2Point b, S2Point c) {
    * a,b,c, except that it has additional logic to make sure that the above
    * properties hold even when the three points are coplanar, and to deal with
    * the limitations of floating-point arithmetic.
+   *
+   *  Note: a, b and c are expected to be of unit length. Otherwise, the results
+   * are undefined.
    */
   public static int robustCCW(S2Point a, S2Point b, S2Point c) {
     return robustCCW(a, b, c, S2Point.crossProd(a, b));
@@ -529,11 +532,12 @@ public static int robustCCW(S2Point a, S2Point b, S2Point c) {
   /**
    * A more efficient version of RobustCCW that allows the precomputed
    * cross-product of A and B to be specified.
+   *
+   *  Note: a, b and c are expected to be of unit length. Otherwise, the results
+   * are undefined
    */
   public static int robustCCW(S2Point a, S2Point b, S2Point c, S2Point aCrossB) {
-    Preconditions.checkArgument(isUnitLength(a));
-    Preconditions.checkArgument(isUnitLength(b));
-    Preconditions.checkArgument(isUnitLength(c));
+    // assert (isUnitLength(a) && isUnitLength(b) && isUnitLength(c));
 
     // There are 14 multiplications and additions to compute the determinant
     // below. Since all three points are normalized, it is possible to show