Reduce isolate startup time in debug mode by 20 millis

runtime/vm/object.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 "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/assembler.h"
 #include "vm/cpu.h"
@@ skipping 17151 matching lines @@
   }
  private:
   uint32_t hash_;
 };
 
 
 void StringHasher::Add(const String& str, intptr_t begin_index, intptr_t len) {
   ASSERT(begin_index >= 0);
   ASSERT(len >= 0);
   ASSERT((begin_index + len) <= str.Length());
+  if (len == 0) {
+    return;
+  }
   if (str.IsOneByteString()) {
+    NoGCScope no_gc;
+    uint8_t* str_addr = OneByteString::CharAddr(str, begin_index);
     for (intptr_t i = 0; i < len; i++) {
-      NoGCScope no_gc;
-      Add(*OneByteString::CharAddr(str, i + begin_index));
+      Add(*str_addr);
+      str_addr++;
     }
   } else {
     String::CodePointIterator it(str, begin_index, len);
     while (it.Next()) {
       Add(it.Current());
     }
   }
 }
 
 
@@ skipping 3374 matching lines @@
   return tag_label.ToCString();
 }
 
 
 void UserTag::PrintJSONImpl(JSONStream* stream, bool ref) const {
   Instance::PrintJSONImpl(stream, ref);
 }
 
 
 }  // namespace dart