Create generic check methods for RangeError causing checks.

sdk/lib/async/stream.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart.async;
 
 // -------------------------------------------------------------------
 // Core Stream types
 // -------------------------------------------------------------------
 
@@ skipping 1147 matching lines @@
    * If an error event occurs before the value is found, the future completes
    * with this error.
    *
    * If a done event occurs before the value is found, the future completes
    * with a [RangeError].
    */
   Future<T> elementAt(int index) {
     if (index is! int || index < 0) throw new ArgumentError(index);
     _Future<T> future = new _Future<T>();
     StreamSubscription subscription;
+    int elementIndex = 0;
     subscription = this.listen(
       (T value) {
-        if (index == 0) {
+        if (index == elementIndex) {
           _cancelAndValue(subscription, future, value);
           return;
         }
-        index -= 1;
+        elementIndex += 1;
       },
       onError: future._completeError,
       onDone: () {
-        future._completeError(new RangeError.value(index));
+        future._completeError(
+            new RangeError.index(index, this, "index", null, elementIndex));
       },
       cancelOnError: true);
     return future;
   }
 
   /**
    * Creates a new stream with the same events as this stream.
    *
    * Whenever more than [timeLimit] passes between two events from this stream,
    * the [onTimeout] function is called.
@@ skipping 468 matching lines @@
 class _ControllerEventSinkWrapper<T> implements EventSink<T> {
   EventSink _sink;
   _ControllerEventSinkWrapper(this._sink);
 
   void add(T data) { _sink.add(data); }
   void addError(error, [StackTrace stackTrace]) {
     _sink.addError(error, stackTrace);
   }
   void close() { _sink.close(); }
 }