Reduce isolate startup time in debug mode by 20 millis

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/object.h"
 #include "vm/object_store.h"
 #include "vm/snapshot.h"
 #include "vm/stub_code.h"
 #include "vm/symbols.h"
 #include "vm/visitor.h"
@@ skipping 1885 matching lines @@
         isolate->current_zone()->Alloc<CharacterType>(len);
     for (intptr_t i = 0; i < len; i++) {
       ptr[i] = reader->Read<CharacterType>();
     }
     *str_obj ^= (*new_symbol)(ptr, len);
   } else {
     // Set up the string object.
     *str_obj = StringType::New(len, HEAP_SPACE(kind));
     str_obj->set_tags(tags);
     str_obj->SetHash(0);  // Will get computed when needed.
+    if (len == 0) {
+      return;
+    }
     NoGCScope no_gc;
+    CharacterType* str_addr = StringType::CharAddr(*str_obj, 0);
     for (intptr_t i = 0; i < len; i++) {
-      *StringType::CharAddr(*str_obj, i) = reader->Read<CharacterType>();
+      *str_addr = reader->Read<CharacterType>();
+      str_addr++;
     }
   }
 }
 
 
 RawOneByteString* OneByteString::ReadFrom(SnapshotReader* reader,
                                           intptr_t object_id,
                                           intptr_t tags,
                                           Snapshot::Kind kind) {
   // Read the length so that we can determine instance size to allocate.
@@ skipping 982 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