Fix a runtime type issue in pkg/analysis_server/benchmark/benchmarks.dart.

Change-Id: I7d73b4cc7c9d43fa9d58d4e2efd7c534c88fbbc7
Reviewed-on: https://dart-review.googlesource.com/64763
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Devon Carew <devoncarew@google.com>
diff --git a/pkg/analysis_server/benchmark/benchmarks.dart b/pkg/analysis_server/benchmark/benchmarks.dart
index d1c7d6d..3bcfb11 100644
--- a/pkg/analysis_server/benchmark/benchmarks.dart
+++ b/pkg/analysis_server/benchmark/benchmarks.dart
@@ -216,8 +216,10 @@
     CompoundBenchMarkResult o = other as CompoundBenchMarkResult;
 
     CompoundBenchMarkResult combined = new CompoundBenchMarkResult(name);
-    List<String> keys =
-        (new Set()..addAll(results.keys)..addAll(o.results.keys)).toList();
+    List<String> keys = (new Set<String>()
+          ..addAll(results.keys)
+          ..addAll(o.results.keys))
+        .toList();
 
     for (String key in keys) {
       combined.add(key, _combine(results[key], o.results[key]));