Create generic check methods for RangeError causing checks.

sdk/lib/collection/set.dart

Index: sdk/lib/collection/set.dart
diff --git a/sdk/lib/collection/set.dart b/sdk/lib/collection/set.dart
index 906312adfed50af1f50d98ad0169f827638f010a..74707611a7bbe6ca32cb97947dbf28bfe0ccb247 100644
--- a/sdk/lib/collection/set.dart
+++ b/sdk/lib/collection/set.dart
@@ -271,13 +271,14 @@ abstract class SetMixin<E> implements Set<E> {
   }
 
   E elementAt(int index) {
-    if (index is! int || index < 0) throw new RangeError.value(index);
-    int remaining = index;
+    if (index is! int) throw new ArgumentError.notNull("index");
+    RangeError.checkNotNegative(index, "index");
+    int elementIndex = 0;
     for (E element in this) {
-      if (remaining == 0) return element;
-      remaining--;
+      if (index == elementIndex) return element;
+      elementIndex++;
     }
-    throw new RangeError.value(index);
+    throw new RangeError.index(index, this, "index", null, elementIndex);
   }
 }