Don't rely on Function being callable in static analysis.

sdk/lib/async/async_error.dart

Index: sdk/lib/async/async_error.dart
diff --git a/sdk/lib/async/async_error.dart b/sdk/lib/async/async_error.dart
index 9a640859b1247dd668c995fc5fa788bea8453703..650d503af8c1ad94bafc135825cf67c4d2a3ca83 100644
--- a/sdk/lib/async/async_error.dart
+++ b/sdk/lib/async/async_error.dart
@@ -9,7 +9,8 @@ _invokeErrorHandler(Function errorHandler,
   if (errorHandler is ZoneBinaryCallback) {
     return errorHandler(error, stackTrace);
   } else {
-    return errorHandler(error);
+    ZoneUnaryCallback unaryErrorHandler = errorHandler;
+    return unaryErrorHandler(error);
   }
 }