Add an external array type to support typed arrays.

runtime/vm/raw_object_snapshot.cc

Index: runtime/vm/raw_object_snapshot.cc
diff --git a/runtime/vm/raw_object_snapshot.cc b/runtime/vm/raw_object_snapshot.cc
index 9a1be618eab884ab160b2025a087f5029beea9a7..f66c0bac420c31df79ab6069f5ae523f8d49a492 100644
--- a/runtime/vm/raw_object_snapshot.cc
+++ b/runtime/vm/raw_object_snapshot.cc
@@ -1247,6 +1247,21 @@ void RawImmutableArray::WriteTo(SnapshotWriter* writer,
 }
 
 
+RawExternalArray* ExternalArray::ReadFrom(SnapshotReader* reader,
+                                          intptr_t object_id,
+                                          bool classes_serialized) {
+  UNIMPLEMENTED();
+  return ExternalArray::null();
+}
+
+
+void RawExternalArray::WriteTo(SnapshotWriter* writer,
+                               intptr_t object_id,
+                               bool serialize_classes) {
+  UNIMPLEMENTED();
+}
+
+
 RawClosure* Closure::ReadFrom(SnapshotReader* reader,
                               intptr_t object_id,
                               bool classes_serialized) {