Make List constructor give better error messages for non-int arguments

sdk/lib/collection/list.dart

Index: sdk/lib/collection/list.dart
diff --git a/sdk/lib/collection/list.dart b/sdk/lib/collection/list.dart
index 9b6b753a815db1dd3f729e54e021322f743bfc58..02e4b711e95b03c5db2549b6032849b01dd1980c 100644
--- a/sdk/lib/collection/list.dart
+++ b/sdk/lib/collection/list.dart
@@ -244,8 +244,12 @@ abstract class ListMixin<E> implements List<E> {
   }
 
   void addAll(Iterable<E> iterable) {
+    int i = this.length;
     for (E element in iterable) {
-      this[this.length++] = element;
+      assert(this.length == i || (throw new ConcurrentModificationError(this)));
+      this.length = i + 1;
+      this[i] = element;
+      i++;
     }
   }