Elements. Keep only AugmentedInstanceElementImpl and subclasses.

Having single implementation of `ClassElement2` (and other elements like it) makes the implementation easier now, and simplifies future changes.

Change-Id: Ifd38a8738fff99b7ac0b387140fdd623be18be01
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/392780
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov

pkg/analyzer/lib/src/dart/analysis/driver.dart

@@ -98,7 +98,7 @@ import 'package:meta/meta.dart';
 // TODO(scheglov): Clean up the list of implicitly analyzed files.
 class AnalysisDriver {
   /// The version of data format, should be incremented on every format change.
-  static const int DATA_VERSION = 409;
+  static const int DATA_VERSION = 410;
 
   /// The number of exception contexts allowed to write. Once this field is
   /// zero, we stop writing any new exception contexts in this process.