[analysis_server/benchmark] Fix memory usage calculation to avoid double-counting.

When several isolates share same isolate group, they share the same heap.
This change iterates over isolate groups and aggregates their heap size instead of iterating over isolates.

Change-Id: I83b04110277b3a2d22ab758b50ee8c4ce480ea7f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/212382
Commit-Queue: Alexander Aprelev <[email protected]>
Reviewed-by: Martin Kustermann <[email protected]>

Author: aam

pkg/analysis_server/benchmark/perf/memory_tests.dart

@@ -254,14 +254,11 @@ mixin ServerMemoryUsageMixin {
 
     var total = 0;
 
-    List isolateRefs = vm['isolates'];
-    for (Map isolateRef in isolateRefs) {
-      var isolate =
-          await service.call('getIsolate', {'isolateId': isolateRef['id']});
-
-      Map _heaps = isolate['_heaps'];
-      total += _heaps['new']['used'] + _heaps['new']['external'] as int;
-      total += _heaps['old']['used'] + _heaps['old']['external'] as int;
+    List isolateGroupsRefs = vm['isolateGroups'];
+    for (Map isolateGroupRef in isolateGroupsRefs) {
+      final heapUsage = await service.call('getIsolateGroupMemoryUsage',
+          {'isolateGroupId': isolateGroupRef['id']});
+      total += heapUsage['heapUsage'] + heapUsage['externalUsage'] as int;
     }
 
     service.dispose();