Remove NullPointerException.

runtime/lib/string.cc

 // Copyright (c) 2011, 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.
 
 #include "vm/bootstrap_natives.h"
 
 #include "vm/exceptions.h"
 #include "vm/native_entry.h"
 #include "vm/object.h"
 #include "vm/symbols.h"
@@ skipping 126 matching lines @@
 }
 
 
 DEFINE_NATIVE_ENTRY(Strings_concatAll, 1) {
   GET_NATIVE_ARGUMENT(Array, strings, arguments->NativeArgAt(0));
   ASSERT(!strings.IsNull());
   // Check that the array contains strings.
   Instance& elem = Instance::Handle();
   for (intptr_t i = 0; i < strings.Length(); i++) {
     elem ^= strings.At(i);
-    if (elem.IsNull()) {
-      GrowableArray<const Object*> args;
-      Exceptions::ThrowByType(Exceptions::kNullPointer, args);
-    }
     if (!elem.IsString()) {
       GrowableArray<const Object*> args;
       args.Add(&elem);
       Exceptions::ThrowByType(Exceptions::kArgument, args);
     }
   }
   return String::ConcatAll(strings);
 }
 
 }  // namespace dart