Replace try/finally with PerformanceTag.makeCurrentWhile().

pkg/analyzer/lib/src/generated/source_io.dart

Index: pkg/analyzer/lib/src/generated/source_io.dart
diff --git a/pkg/analyzer/lib/src/generated/source_io.dart b/pkg/analyzer/lib/src/generated/source_io.dart
index ac08551871be087fefbf58cc18e942da9027dd5a..06211df7252cde81872ea95a13dd93847f5ef585 100644
--- a/pkg/analyzer/lib/src/generated/source_io.dart
+++ b/pkg/analyzer/lib/src/generated/source_io.dart
@@ -129,12 +129,9 @@ class FileBasedSource extends Source {
 
   @override
   TimestampedData<String> get contents {
-    PerformanceTag prevTag = PerformanceStatistics.io.makeCurrent();
-    try {
+    return PerformanceStatistics.io.makeCurrentWhile(() {
       return contentsFromFile;
-    } finally {
-      prevTag.makeCurrent();
-    }
+    });
   }
 
   /**