Add Dart_LibraryId method unblocking landing 45613003. TBR

runtime/include/dart_mirrors_api.h

Index: runtime/include/dart_mirrors_api.h
diff --git a/runtime/include/dart_mirrors_api.h b/runtime/include/dart_mirrors_api.h
index 3a4c9ce772a6bfa68e9e965b3040dcd4470fb7d3..be50e49ac4f224e844ab06a3c98d9b777032a915 100644
--- a/runtime/include/dart_mirrors_api.h
+++ b/runtime/include/dart_mirrors_api.h
@@ -122,6 +122,14 @@ DART_EXPORT Dart_Handle Dart_FunctionIsSetter(Dart_Handle function,
 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.
  *