log warnings to stderr not stdout

features/retry.feature

@@ -26,7 +26,7 @@ Feature: Retry flaky tests
       Given(/^a step$/, function() {})
       """
     When I run cucumber-js with `--retry 1 --retryTagFilter @flaky`
-    Then the output contains the text:
+    Then the error output contains the text:
       """
       the argument --retryTagFilter is deprecated and will be removed in a future release; please use --retry-tag-filter
       """

features/support/world.ts

@@ -60,25 +60,31 @@ export class World {
       })
     } else {
       const stdout = new PassThrough()
+      const stderr = new PassThrough()
       const cli = new Cli({
         argv: args,
         cwd,
         stdout,
+        stderr,
       })
-      let error: any, stderr: string
+      let error: any
       try {
         const { success } = await cli.run()
         if (!success) {
           error = new Error('CLI exited with non-zero')
           error.code = 42
         }
-        stderr = ''
       } catch (err) {
         error = err
-        stderr = VError.fullStack(error)
+        stderr.write(VError.fullStack(error))
       }
       stdout.end()
-      result = { error, stdout: await toString(stdout), stderr }
+      stderr.end()
+      result = {
+        error,
+        stdout: await toString(stdout),
+        stderr: await toString(stderr),
+      }
     }
     const envelopes: messages.Envelope[] = []
     const messageOutputPath = path.join(cwd, messageFilename)

src/cli/index.ts

@@ -45,11 +45,11 @@ export default class Cli {
   private readonly stdout: IFormatterStream
   private readonly console: Console
 
-  constructor({ argv, cwd, stdout }) {
+  constructor({ argv, cwd, stdout, stderr }) {
     this.argv = argv
     this.cwd = cwd
     this.stdout = stdout
-    this.console = new Console(stdout)
+    this.console = new Console(stderr)
   }
 
   async getConfiguration(): Promise<IConfiguration> {

src/cli/run.ts

@@ -12,6 +12,7 @@ export default async function run(): Promise<void> {
     argv: process.argv,
     cwd,
     stdout: process.stdout,
+    stderr: process.stderr,
   })
 
   let result: ICliRunResult