Implement external strings.

runtime/include/dart_api.h

Index: runtime/include/dart_api.h
diff --git a/runtime/include/dart_api.h b/runtime/include/dart_api.h
index bf89f60b2ab4bba2481e54e181e196ae796d557b..3fddb54e7f781f030ab4ef0c25d7073de80f0a24 100755
--- a/runtime/include/dart_api.h
+++ b/runtime/include/dart_api.h
@@ -762,6 +762,24 @@ DART_EXPORT Dart_Handle Dart_NewString16(const uint16_t* codepoints,
 DART_EXPORT Dart_Handle Dart_NewString32(const uint32_t* codepoints,
                                          intptr_t length);
 
+
+typedef void (*Dart_PeerFinalizer)(void* peer);
+
+DART_EXPORT Dart_Handle Dart_NewExternalString8(const uint8_t* codepoints,
+                                                intptr_t length,
+                                                void* peer,
+                                                Dart_PeerFinalizer callback);
+
+DART_EXPORT Dart_Handle Dart_NewExternalString16(const uint16_t* codepoints,
+                                                 intptr_t length,
+                                                 void* peer,
+                                                 Dart_PeerFinalizer callback);
+
+DART_EXPORT Dart_Handle Dart_NewExternalString32(const uint32_t* codepoints,
+                                                 intptr_t length,
+                                                 void* peer,
+                                                 Dart_PeerFinalizer callback);
+
 /**
  * Gets the codepoints from a String.
  *