Optimize stores to ExternalUint8Array and ExternalUint8ClampedArray in the optimizer.

tests/standalone/byte_array_test.dart

 // Copyright (c) 2012, 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.
 //
 // Dart test program for testing native byte arrays.
 
 // Library tag to be able to run in html test framework.
 library ByteArrayTest;
 
 import 'dart:scalarlist';
@@ skipping 36 matching lines @@
   Expect.isTrue(externalClampedByteArray is Uint8ClampedList);
   Expect.isFalse(externalClampedByteArray is Uint8List);
   Expect.equals(0, externalClampedByteArray.length);
   Expect.equals(0, externalClampedByteArray.lengthInBytes());
 
   externalClampedByteArray = new Uint8ClampedList.transferable(10);
   Expect.equals(10, externalClampedByteArray.length);
   for (int i = 0; i < 10; i++) {
     Expect.equals(0, externalClampedByteArray[i]);
   }
-
+  for (int i = 0; i < 10; i++) {
+    externalClampedByteArray[i] = i + 250;
+  }
+  for (int i = 0; i < 10; i++) {
+    Expect.equals(i + 250 > 255 ? 255 : i + 250, externalClampedByteArray[i]);
+  }
 }
 
 void testUnsignedByteArrayRange(bool check_throws) {
   Uint8List byteArray;
   byteArray = new Uint8List(10);
 
   byteArray[1] = 255;
   Expect.equals(255, byteArray[1]);
   byteArray[1] = 0;
   Expect.equals(0, byteArray[1]);
@@ skipping 203 matching lines @@
     testSetRange();
     testIndexOutOfRange();
     testIndexOf();
     testSubArray();
   }
   testByteArrayRange(true);
   testUnsignedByteArrayRange(true);
   testExternalClampedUnsignedByteArrayRange(true);
 }