Remove deprecated parts of dart:async.

utils/apidoc/apidoc.dart

Index: utils/apidoc/apidoc.dart
diff --git a/utils/apidoc/apidoc.dart b/utils/apidoc/apidoc.dart
index fe4e617b5be906f53d59124ee7fbc0becc4878b9..2f5f72d9603437e1d55d1a70f4caa1cea812757f 100644
--- a/utils/apidoc/apidoc.dart
+++ b/utils/apidoc/apidoc.dart
@@ -182,9 +182,8 @@ void main() {
             packageRoot))
       .then((_) => compileScript(mode, outputDir, libPath, apidoc.tmpPath))
       .then((_) => print(apidoc.status))
-      .catchError((e) {
+      .catchError((e, trace) {
         print('Error: generation failed: ${e}');
-        var trace = getAttachedStackTrace(e);
         if (trace != null) print("StackTrace: $trace");
         apidoc.cleanup();
         exit(1);