Fix for issue 23598, add a Dart_CreateLibrarySnapshot entrypoint to the API.

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 89131ff02ca6abec9cb6eb73e13d4f21a02b9f16..027b9379c8369f331d0705e69e0d02b3f84e84d6 100755
--- a/runtime/include/dart_api.h
+++ b/runtime/include/dart_api.h
@@ -1056,6 +1056,27 @@ DART_EXPORT Dart_Handle Dart_CreateScriptSnapshot(uint8_t** buffer,
                                                   intptr_t* size);
 
 /**
+ * Creates a snapshot of the specified library loaded in the isolate.
+ *
+ * A library snapshot can be used for implementing fast startup of applications
+ * (skips tokenizing and parsing process). A Snapshot of the library
+ * can only be created before any dart code has executed.
+ *
+ * Requires there to be a current isolate which already has loaded the library.
+ *
+ * \param library A library for which the snapshot needs to be created.
+ * \param buffer Returns a pointer to a buffer containing
+ *   the snapshot. This buffer is scope allocated and is only valid
+ *   until the next call to Dart_ExitScope.
+ * \param size Returns the size of the buffer.
+ *
+ * \return A valid handle if no error occurs during the operation.
+ */
+DART_EXPORT Dart_Handle Dart_CreateLibrarySnapshot(Dart_Handle library,
+                                                   uint8_t** buffer,
+                                                   intptr_t* size);
+
+/**
  * Schedules an interrupt for the specified isolate.
  *
  * When the isolate is interrupted, the isolate interrupt callback