Split S2Edge and UndirectedEdge up to make equals() method correct.

Before this CL it was possible to have instances of S2Edge such that:

a.equals(b) ==/==> b.equals(a)
b.equals(a) && b.equals(c) ==/==> a.equals(c)

Undirected edges are just fundamentally a different type to an S2Edge
and should be treated as such. Additionally they are only used privately
in one place and never using polymorphic behaviour.
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=25140523

Author: hagbard

src/com/google/common/geometry/S2Edge.java

@@ -21,7 +21,7 @@
  *
  * @author kirilll@google.com (Kirill Levin)
  */
-public class S2Edge {
+public final class S2Edge {
 
   private final S2Point start;
   private final S2Point end;
@@ -41,15 +41,15 @@ public S2Point getEnd() {
 
   @Override
   public String toString() {
-    return "Edge: (" + start.toDegreesString() + " -> " + end.toDegreesString() + ")\n" + "   or ["
-        + start + " -> " + end + "]";
+    return String.format("Edge: (%s -> %s)\n   or [%s -> %s]",
+        start.toDegreesString(), end.toDegreesString(), start, end);
   }
 
   @Override
   public int hashCode() {
     return getStart().hashCode() - getEnd().hashCode();
   }
-  
+
   @Override
   public boolean equals(Object o) {
     if (o == null || !(o instanceof S2Edge)) {

src/com/google/common/geometry/S2Polygon.java

@@ -1084,10 +1084,32 @@ public String toString() {
     return sb.toString();
   }
 
-  private static class UndirectedEdge extends S2Edge {
+  private static final class UndirectedEdge {
+    // Note: An UndirectedEdge and an S2Edge can never be considered equal (in
+    // terms of the equals() method) and hence they re not be related types.
+    // If you need to convert between the types then separate conversion
+    // methods should be introduced.
 
-    public UndirectedEdge(S2Point vertexA, S2Point vertexB) {
-      super(vertexA, vertexB);
+    private final S2Point a;
+    private final S2Point b;
+
+    public UndirectedEdge(S2Point start, S2Point end) {
+      this.a = start;
+      this.b = end;
+    }
+
+    public S2Point getStart() {
+      return a;
+    }
+
+    public S2Point getEnd() {
+      return b;
+    }
+
+    @Override
+    public String toString() {
+      return String.format("Edge: (%s <-> %s)\n   or [%s <-> %s]",
+          a.toDegreesString(), b.toDegreesString(), a, b);
     }
 
     @Override