Use unreachable in snapshot generation when we do not expect the object to be sent in a message.

vm/raw_object_snapshot.cc

 // 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.
 
 #include "vm/bigint_operations.h"
 #include "vm/object.h"
 #include "vm/object_store.h"
 #include "vm/snapshot.h"
 #include "vm/symbols.h"
 #include "vm/visitor.h"
@@ skipping 1128 matching lines @@
                        intptr_t object_id,
                        Snapshot::Kind kind) {
   UNREACHABLE();
 }
 
 
 RawApiError* ApiError::ReadFrom(SnapshotReader* reader,
                                 intptr_t object_id,
                                 intptr_t tags,
                                 Snapshot::Kind kind) {
-  UNIMPLEMENTED();
+  UNREACHABLE();
   return ApiError::null();
 }
 
 
 void RawApiError::WriteTo(SnapshotWriter* writer,
                           intptr_t object_id,
                           Snapshot::Kind kind) {
-  UNIMPLEMENTED();
+  UNREACHABLE();
 }
 
 
 RawLanguageError* LanguageError::ReadFrom(SnapshotReader* reader,
                                           intptr_t object_id,
                                           intptr_t tags,
                                           Snapshot::Kind kind) {
-  UNIMPLEMENTED();
+  UNREACHABLE();
   return LanguageError::null();
 }
 
 
 void RawLanguageError::WriteTo(SnapshotWriter* writer,
                                intptr_t object_id,
                                Snapshot::Kind kind) {
-  UNIMPLEMENTED();
+  UNREACHABLE();
 }
 
 
 RawUnhandledException* UnhandledException::ReadFrom(SnapshotReader* reader,
                                                     intptr_t object_id,
                                                     intptr_t tags,
                                                     Snapshot::Kind kind) {
-  UNIMPLEMENTED();
+  UNREACHABLE();
   return UnhandledException::null();
 }
 
 
 void RawUnhandledException::WriteTo(SnapshotWriter* writer,
                                     intptr_t object_id,
                                     Snapshot::Kind kind) {
-  UNIMPLEMENTED();
+  UNREACHABLE();
 }
 
 
 RawUnwindError* UnwindError::ReadFrom(SnapshotReader* reader,
                                       intptr_t object_id,
                                       intptr_t tags,
                                       Snapshot::Kind kind) {
-  UNIMPLEMENTED();
+  UNREACHABLE();
   return UnwindError::null();
 }
 
 
 void RawUnwindError::WriteTo(SnapshotWriter* writer,
                              intptr_t object_id,
                              Snapshot::Kind kind) {
-  UNIMPLEMENTED();
+  UNREACHABLE();
 }
 
 
 RawInstance* Instance::ReadFrom(SnapshotReader* reader,
                                 intptr_t object_id,
                                 intptr_t tags,
                                 Snapshot::Kind kind) {
   UNREACHABLE();
   return Instance::null();
 }
@@ skipping 867 matching lines @@
   // Write out the class and tags information.
   writer->WriteIndexedObject(kWeakPropertyCid);
   writer->WriteIntptrValue(writer->GetObjectTags(this));
 
   // Write out all the other fields.
   writer->Write<RawObject*>(ptr()->key_);
   writer->Write<RawObject*>(ptr()->value_);
 }
 
 }  // namespace dart