Change IllegalArgumentException to ArgumentError.

runtime/lib/byte_array.dart

Index: runtime/lib/byte_array.dart
diff --git a/runtime/lib/byte_array.dart b/runtime/lib/byte_array.dart
index 59b878cfc9ccbcbc952fb5fd575dbfd4d85e4306..0897917bb0d6396229e1897b9c53fe84e5209ca7 100644
--- a/runtime/lib/byte_array.dart
+++ b/runtime/lib/byte_array.dart
@@ -323,7 +323,7 @@ int _requireInteger(object) {
   if (object is int) {
     return object;
   }
-  throw new IllegalArgumentException("$object is not an integer");
+  throw new ArgumentError("$object is not an integer");
 }
 
 
@@ -334,7 +334,7 @@ int _requireIntegerOrNull(object, value) {
   if (object === null) {
     return _requireInteger(value);
   }
-  throw new IllegalArgumentException("$object is not an integer or null");
+  throw new ArgumentError("$object is not an integer or null");
 }