Always set inserted range in List.insertRange.

runtime/lib/growable_array.dart

Index: runtime/lib/growable_array.dart
diff --git a/runtime/lib/growable_array.dart b/runtime/lib/growable_array.dart
index eb907689184bebace5c14fcb87bed3b74d041324..772bec094e769e32a610ac7cb4b0d07adc5184db 100644
--- a/runtime/lib/growable_array.dart
+++ b/runtime/lib/growable_array.dart
@@ -49,10 +49,8 @@ class GrowableObjectArray<T> implements List<T> {
                 this,
                 start + length,
                 old_length - start);
-    if (initialValue !== null) {
-      for (int i = start; i < start + length; i++) {
-        this[i] = initialValue;
-      }
+    for (int i = start; i < start + length; i++) {
+      this[i] = initialValue;
     }
   }