Add Dart_LibraryId method unblocking landing 45613003. TBR

runtime/include/dart_mirrors_api.h

 /*
  * Copyright (c) 2013, 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 INCLUDE_DART_MIRRORS_API_H_
 #define INCLUDE_DART_MIRRORS_API_H_
 
 #include "include/dart_api.h"
@@ skipping 104 matching lines @@
  */
 DART_EXPORT Dart_Handle Dart_FunctionIsSetter(Dart_Handle function,
                                               bool* is_setter);
 
 /**
  * Returns the name of a library as declared in the #library directive.
  */
 DART_EXPORT Dart_Handle Dart_LibraryName(Dart_Handle library);
 
 /**
+ * Returns in \library_id the library id of the given \library.
+ *
+ * \return A valid handle if no error occurs during the operation.
+ */
+DART_EXPORT Dart_Handle Dart_LibraryId(Dart_Handle library,
+                                       intptr_t* library_id);
+
+/**
  * Returns a list of the names of all classes and interfaces declared
  * in a library.
  *
  * \return If no error occurs, a list of strings is returned.
  *   Otherwise an error handle is returned.
  */
 DART_EXPORT Dart_Handle Dart_LibraryGetClassNames(Dart_Handle library);
 
 /**
  * Retrieves the function of a closure.
  *
  * \return A handle to the function of the closure, or an error handle if the
  *   argument is not a closure.
  */
 DART_EXPORT Dart_Handle Dart_ClosureFunction(Dart_Handle closure);
 
 
 #endif  /* INCLUDE_DART_MIRRORS_API_H_ */  /* NOLINT */