Remove AsyncError with Expando.

sdk/lib/_internal/dartdoc/lib/dartdoc.dart

Index: sdk/lib/_internal/dartdoc/lib/dartdoc.dart
diff --git a/sdk/lib/_internal/dartdoc/lib/dartdoc.dart b/sdk/lib/_internal/dartdoc/lib/dartdoc.dart
index 38d17063037edc0e77d4090da0fa1e9d01bfc293..9057b6e8a45660a1e3e1fda3bbe00629f3754d91 100644
--- a/sdk/lib/_internal/dartdoc/lib/dartdoc.dart
+++ b/sdk/lib/_internal/dartdoc/lib/dartdoc.dart
@@ -126,7 +126,7 @@ Future copyDirectory(Path from, Path to) {
         }
       },
       onDone: () => completer.complete(),
-      onError: (e) => completer.completeError(e.error, e.stackTrace));
+      onError: (e) => completer.completeError(e));
   return completer.future;
 }
 
@@ -163,8 +163,10 @@ void _compileScript() {
       });
     }).then((_) {
       replyTo.send(['success']);
-    }).catchError((e) {
-      replyTo.send(['error', e.error.toString(), e.stackTrace.toString()]);
+    }).catchError((error) {
+      var trace = getAttachedStackTrace(error);
+      var traceString = trace == null ? "" : trace.toString();
+      replyTo.send(['error', error.toString(), traceString]);
     });
   });
 }