Wait for stderr in `runCommandAndStreamOutput()` (#9598)

packages/flutter_tools/lib/src/base/process.dart

@@ -102,14 +102,13 @@ Future<Process> runCommand(List<String> cmd, {
   String workingDirectory,
   bool allowReentrantFlutter: false,
   Map<String, String> environment
-}) async {
+}) {
   _traceCommand(cmd, workingDirectory: workingDirectory);
-  final Process process = await processManager.start(
+  return processManager.start(
     cmd,
     workingDirectory: workingDirectory,
-    environment: _environment(allowReentrantFlutter, environment)
+    environment: _environment(allowReentrantFlutter, environment),
   );
-  return process;
 }
 
 /// This runs the command and streams stdout/stderr from the child process to
@@ -129,7 +128,7 @@ Future<int> runCommandAndStreamOutput(List<String> cmd, {
     allowReentrantFlutter: allowReentrantFlutter,
     environment: environment
   );
-  final StreamSubscription<String> subscription = process.stdout
+  final StreamSubscription<String> stdoutSubscription = process.stdout
     .transform(UTF8.decoder)
     .transform(const LineSplitter())
     .where((String line) => filter == null ? true : filter.hasMatch(line))
@@ -144,7 +143,7 @@ Future<int> runCommandAndStreamOutput(List<String> cmd, {
           printStatus(message);
       }
     });
-  process.stderr
+  final StreamSubscription<String> stderrSubscription = process.stderr
     .transform(UTF8.decoder)
     .transform(const LineSplitter())
     .where((String line) => filter == null ? true : filter.hasMatch(line))
@@ -157,8 +156,12 @@ Future<int> runCommandAndStreamOutput(List<String> cmd, {
 
   // Wait for stdout to be fully processed
   // because process.exitCode may complete first causing flaky tests.
-  await subscription.asFuture<Null>();
-  subscription.cancel();
+  await Future.wait(<Future<Null>>[
+    stdoutSubscription.asFuture<Null>(),
+    stderrSubscription.asFuture<Null>(),
+  ]);
+  stdoutSubscription.cancel();
+  stderrSubscription.cancel();
 
   return await process.exitCode;
 }