Use AnalysisContextCollection for DartServerPlugin.

Not for landing, has debug logging.

There is also an example (also not for landing) of a concrete plugin.

Change-Id: Id79336205b7b6d4cd462e7cfa0fdc7bea41c2987
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/243161
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

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

@@ -563,7 +563,6 @@ class AnalysisDriver implements AnalysisDriverGeneric {
   /// library that was resynthesized, but after some initial analysis we might
   /// not get again to many of these libraries. So, we should clear the context
   /// periodically.
-  @visibleForTesting
   void clearLibraryContext() {
     _libraryContext?.dispose();
     _libraryContext = null;

pkg/analyzer_plugin/CHANGELOG.md

@@ -1,3 +1,6 @@
+## 0.11.0
+- Using `AnalysisContextCollection` and `AnalysisContext` for analysis.
+
 ## 0.10.0
 - Support version `4.x` of the `analyzer` package