[analyzer] Don't collect analytics if not using it; cache getTopLevelDeclarations results in DartFixContext

Sometimes - on responding to `edit.getFixes` requests, the method
`getTopLevelDeclarations` is called several times with the same
parameter and caching has then sometimes sped things up.

I then also noticed that it caused `reportAnalysisAnalytics` to be
called many times (e.g. 73 times for one `edit.getFixes` request)
each time spending time collecting data which - on passing to
`analyticsManager.analysisComplete` - was just thrown away because it
only saves the first one.

Combined these changes have been observed to reduce the response time
on `edit.getFixes` by up (down?) to ~20% (e.g. from 5 seconds
to 1 second).

In response to http://b/407797012 where more data is also available.

Unfortunately I haven't been able to reproduce this is a benchmark
(yet anyway). I don't know if it needs slower slower reads, an extreme
amount of files, several contexts or something else entirely.

Change-Id: Id214b0e1b7d8bbef7f6d956408f17fab45618b78
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/420323
Reviewed-by: Phil Quitslund <[email protected]>
Commit-Queue: Johnni Winther <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: jensjoha

pkg/analysis_server/lib/src/analysis_server.dart

@@ -917,6 +917,9 @@ abstract class AnalysisServer {
   /// Report analytics data related to the number and size of files that were
   /// analyzed.
   void reportAnalysisAnalytics() {
+    if (!analyticsManager.needsAnslysisCompleteCall()) {
+      return;
+    }
     var packagesFileMap = <String, File?>{};
     var immediateFileCount = 0;
     var immediateFileLineCount = 0;

pkg/analysis_server/lib/src/analytics/analytics_manager.dart

@@ -220,6 +220,8 @@ class AnalyticsManager {
     requestData.addValue(openWorkspacePathsKey, openWorkspacePaths.length);
   }
 
+  bool needsAnslysisCompleteCall() => _contextStructure == null;
+
   Future<void> sendMemoryUsage(MemoryUsageEvent event) async {
     var delta = event.delta;
     var seconds = event.period?.inSeconds;

pkg/analysis_server_plugin/lib/edit/fix/dart_fix_context.dart

@@ -40,6 +40,14 @@ class DartFixContext implements FixContext {
   /// The workspace in which the fix contributor operates.
   final ChangeWorkspace workspace;
 
+  /// Cache of previously computed [getTopLevelDeclarations] responses.
+  ///
+  /// It's been observed that the same request is fired multiple times for at
+  /// least some getFixes requsts. Caching the response can speed up such
+  /// requests.
+  final Map<String, Future<Map<LibraryElement2, Element2>>>
+      _cachedTopLevelDeclarations = {};
+
   @override
   final AnalysisError error;
 
@@ -58,7 +66,11 @@ class DartFixContext implements FixContext {
   ///
   /// For getters and setters the corresponding top-level variable is returned.
   Future<Map<LibraryElement2, Element2>> getTopLevelDeclarations(String name) {
-    return TopLevelDeclarations(unitResult).withName(name);
+    var cachedResult = _cachedTopLevelDeclarations[name];
+    if (cachedResult != null) return cachedResult;
+    var result = TopLevelDeclarations(unitResult).withName(name);
+    _cachedTopLevelDeclarations[name] = result;
+    return result;
   }
 
   /// Returns libraries with extensions that declare non-static public