Make StringBase._substringUnchecked a native function.

runtime/vm/bootstrap_natives.h

 // 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.
 
 #ifndef VM_BOOTSTRAP_NATIVES_H_
 #define VM_BOOTSTRAP_NATIVES_H_
 
 #include "vm/native_entry.h"
 
 // bootstrap dart natives used in the core dart library.
@@ skipping 51 matching lines @@
   V(JSSyntaxRegExp_multiLine, 1)                                               \
   V(JSSyntaxRegExp_ignoreCase, 1)                                              \
   V(JSSyntaxRegExp_getGroupCount, 1)                                           \
   V(JSSyntaxRegExp_ExecuteMatch, 3)                                            \
   V(ObjectArray_allocate, 2)                                                   \
   V(ObjectArray_getIndexed, 2)                                                 \
   V(ObjectArray_setIndexed, 3)                                                 \
   V(ObjectArray_getLength, 1)                                                  \
   V(ObjectArray_copyFromObjectArray, 5)                                        \
   V(StringBase_createFromCodePoints, 1)                                        \
+  V(StringBase_substringUnchecked, 3)                                          \
   V(String_hashCode, 1)                                                        \
   V(String_getLength, 1)                                                       \
   V(String_charAt, 2)                                                          \
   V(String_charCodeAt, 2)                                                      \
   V(String_concat, 2)                                                          \
   V(String_toLowerCase, 1)                                                     \
   V(String_toUpperCase, 1)                                                     \
   V(Strings_concatAll, 1)                                                      \
   V(MathNatives_sqrt, 1)                                                       \
   V(MathNatives_sin, 1)                                                        \
@@ skipping 111 matching lines @@
   V(WeakProperty_new, 2)                                                       \
   V(WeakProperty_getKey, 1)                                                    \
   V(WeakProperty_getValue, 1)                                                  \
   V(WeakProperty_setValue, 2)                                                  \
 
 BOOTSTRAP_NATIVE_LIST(DECLARE_NATIVE_ENTRY)
 
 }  // namespace dart
 
 #endif  // VM_BOOTSTRAP_NATIVES_H_