[analysis_server] Fix collect report returning 500 because of a crash on json convert

After 816b974 the download report on
"Analysis Server Diagnostics" did not work.

Change-Id: I14f2525730772f7cf72a7a9c622ae3094602766a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/434680
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Jens Johansen <[email protected]>

Author: jensjoha

pkg/analysis_server/lib/src/status/pages/collect_report_page.dart

@@ -166,9 +166,10 @@ class CollectReportPage extends DiagnosticPage {
       for (var cycle in cycles) {
         cycleData[cycle.size] = (cycleData[cycle.size] ?? 0) + 1;
       }
-      var sortedCycleData = <int, int>{};
+      // Json maps need string keys.
+      var sortedCycleData = <String, int>{};
       for (var size in cycleData.keys.toList()..sort()) {
-        sortedCycleData[size] = cycleData[size]!;
+        sortedCycleData['$size'] = cycleData[size]!;
       }
       contextData['libraryCycleData'] = sortedCycleData;
     }