Avoid allocating an exception object in the snapshot reader.

runtime/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/stub_code.h"
 #include "vm/symbols.h"
@@ skipping 1541 matching lines @@
   // Write out all the object pointer fields.
   SnapshotWriterVisitor visitor(writer);
   visitor.VisitPointers(from(), to());
 }
 
 
 RawUnhandledException* UnhandledException::ReadFrom(SnapshotReader* reader,
                                                     intptr_t object_id,
                                                     intptr_t tags,
                                                     Snapshot::Kind kind) {
-  UNREACHABLE();
-  return UnhandledException::null();
+  UnhandledException& result = UnhandledException::ZoneHandle(
+      reader->isolate(), NEW_OBJECT(UnhandledException));
+  reader->AddBackRef(object_id, &result, kIsDeserialized);
+
+  // Set the object tags.
+  result.set_tags(tags);
+
+  // Set all the object fields.
+  // TODO(5411462): Need to assert No GC can happen here, even though
+  // allocations may happen.
+  intptr_t num_flds = (result.raw()->to() - result.raw()->from());
+  for (intptr_t i = 0; i <= num_flds; i++) {
+    *(result.raw()->from() + i) = reader->ReadObjectRef();
+  }
+  return result.raw();
 }
 
 
 void RawUnhandledException::WriteTo(SnapshotWriter* writer,
                                     intptr_t object_id,
                                     Snapshot::Kind kind) {
-  UNREACHABLE();
+  // Write out the serialization header value for this object.
+  writer->WriteInlinedObjectHeader(object_id);
+
+  // Write out the class and tags information.
+  writer->WriteVMIsolateObject(kUnhandledExceptionCid);
+  writer->WriteTags(writer->GetObjectTags(this));
+  // Write out all the object pointer fields.
+  SnapshotWriterVisitor visitor(writer);
+  visitor.VisitPointers(from(), to());
 }
 
 
 RawUnwindError* UnwindError::ReadFrom(SnapshotReader* reader,
                                       intptr_t object_id,
                                       intptr_t tags,
                                       Snapshot::Kind kind) {
   UNREACHABLE();
   return UnwindError::null();
 }
@@ skipping 1218 matching lines @@
     // We do not allow objects with native fields in an isolate message.
     writer->SetWriteException(Exceptions::kArgument,
                               "Illegal argument in isolate message"
                               " : (object is a UserTag)");
   } else {
     UNREACHABLE();
   }
 }
 
 }  // namespace dart