Add Dart class for _ExternalUint8ClampedArray and API to allocate it.

runtime/vm/bootstrap_natives.h

 // 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.
 
 #ifndef VM_BOOTSTRAP_NATIVES_H_
 #define VM_BOOTSTRAP_NATIVES_H_
 
 #include "vm/native_entry.h"
 
 // bootstrap dart natives used in the core dart library.
@@ skipping 160 matching lines @@
   V(Float32Array_getIndexed, 2)                                                \
   V(Float32Array_setIndexed, 3)                                                \
   V(Float64Array_new, 1)                                                       \
   V(Float64Array_newTransferable, 1)                                           \
   V(Float64Array_getIndexed, 2)                                                \
   V(Float64Array_setIndexed, 3)                                                \
   V(ExternalInt8Array_getIndexed, 2)                                           \
   V(ExternalInt8Array_setIndexed, 3)                                           \
   V(ExternalUint8Array_getIndexed, 2)                                          \
   V(ExternalUint8Array_setIndexed, 3)                                          \
+  V(ExternalUint8ClampedArray_getIndexed, 2)                                   \
+  V(ExternalUint8ClampedArray_setIndexed, 3)                                   \
   V(ExternalInt16Array_getIndexed, 2)                                          \
   V(ExternalInt16Array_setIndexed, 3)                                          \
   V(ExternalUint16Array_getIndexed, 2)                                         \
   V(ExternalUint16Array_setIndexed, 3)                                         \
   V(ExternalInt32Array_getIndexed, 2)                                          \
   V(ExternalInt32Array_setIndexed, 3)                                          \
   V(ExternalUint32Array_getIndexed, 2)                                         \
   V(ExternalUint32Array_setIndexed, 3)                                         \
   V(ExternalInt64Array_getIndexed, 2)                                          \
   V(ExternalInt64Array_setIndexed, 3)                                          \
@@ skipping 34 matching lines @@
   static void DN_##name(Dart_NativeArguments args);
 
   BOOTSTRAP_NATIVE_LIST(DECLARE_BOOTSTRAP_NATIVE)
 
 #undef DECLARE_BOOTSTRAP_NATIVE
 };
 
 }  // namespace dart
 
 #endif  // VM_BOOTSTRAP_NATIVES_H_