Fix to asiva's code review comments on TBR Cl.

runtime/vm/debugger_api_impl.cc

Index: runtime/vm/debugger_api_impl.cc
diff --git a/runtime/vm/debugger_api_impl.cc b/runtime/vm/debugger_api_impl.cc
index 3ffff4a21679cd219f095dbeb5ac19346385d69b..c20bf670a12307a11a992be55ababd04ab5b44e7 100644
--- a/runtime/vm/debugger_api_impl.cc
+++ b/runtime/vm/debugger_api_impl.cc
@@ -796,6 +796,22 @@ DART_EXPORT Dart_Handle Dart_GetLibraryFromId(intptr_t library_id) {
 }
 
 
+DART_EXPORT Dart_Handle Dart_LibraryId(Dart_Handle library,
+                                       intptr_t* library_id) {
+  Isolate* isolate = Isolate::Current();
+  DARTSCOPE(isolate);
+  const Library& lib = Api::UnwrapLibraryHandle(isolate, library);
+  if (lib.IsNull()) {
+    RETURN_TYPE_ERROR(isolate, library, Library);
+  }
+  if (library_id == NULL) {
+    RETURN_NULL_ERROR(library_id);
+  }
+  *library_id = lib.index();
+  return Api::Success();
+}
+
+
 DART_EXPORT Dart_Handle Dart_GetLibraryImports(intptr_t library_id) {
   Isolate* isolate = Isolate::Current();
   ASSERT(isolate != NULL);