Elements. Remove metadata3, store MetadataImpl directly.

This CL continues https://dart-review.googlesource.com/c/sdk/+/430261

Change-Id: I4f717dc25f84571e6f86a470e6ddfcf452928682
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/432161
Commit-Queue: Konstantin Shcheglov <[email protected]>
Reviewed-by: Paul Berry <[email protected]>

Author: scheglov

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

@@ -110,7 +110,7 @@ testFineAfterLibraryAnalyzerHook;
 // 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 = 463;
+  static const int DATA_VERSION = 464;
 
   /// The number of exception contexts allowed to write. Once this field is
   /// zero, we stop writing any new exception contexts in this process.