Optimize stores to ExternalUint8Array and ExternalUint8ClampedArray in the optimizer.

tests/standalone/byte_array_test.dart

Index: tests/standalone/byte_array_test.dart
===================================================================
--- tests/standalone/byte_array_test.dart	(revision 18309)
+++ tests/standalone/byte_array_test.dart	(working copy)
@@ -54,7 +54,12 @@
   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) {