Add version validation for LookupMap, also add unittest directly in LookupMap (take 2)

pkg/compiler/lib/src/common/backend_api.dart

Index: pkg/compiler/lib/src/common/backend_api.dart
diff --git a/pkg/compiler/lib/src/common/backend_api.dart b/pkg/compiler/lib/src/common/backend_api.dart
index e715f29ab36996a46df81950816a11811e906a32..42ad0903cf4db77c38a4a37c7b73617012a9e18b 100644
--- a/pkg/compiler/lib/src/common/backend_api.dart
+++ b/pkg/compiler/lib/src/common/backend_api.dart
@@ -343,6 +343,9 @@ abstract class Backend {
   /// times, but [onQueueClosed] is only called once.
   void onQueueClosed() {}
 
+  /// Called when the compiler starts running the codegen enqueuer.
+  void onCodegenStart() {}
+
   /// Called after [element] has been resolved.
   // TODO(johnniwinther): Change [TreeElements] to [Registry] or a dependency
   // node. [elements] is currently unused by the implementation.