Fix missing concurrency check in [].forEach.

runtime/lib/array.dart

Index: runtime/lib/array.dart
diff --git a/runtime/lib/array.dart b/runtime/lib/array.dart
index 6e038881bc3c84926de4b35d1afdb483ce7e88ec..bb68f0b3b83c1ab79b32aaedef4741681377739e 100644
--- a/runtime/lib/array.dart
+++ b/runtime/lib/array.dart
@@ -67,7 +67,7 @@ class _ObjectArray<E> implements List<E> {
     if (start < 0 || start > this.length) {
       throw new RangeError.range(start, 0, this.length);
     }
-    if (end < 0 || end > this.length) {
+    if (end < start || end > this.length) {
       throw new RangeError.range(end, start, this.length);
     }
     int length = end - start;