Attach standard streams of the child sky_shell process so that logs from the shell show up. (#4024)

packages/flutter_tools/lib/src/test/flutter_platform.dart

@@ -54,6 +54,18 @@ Future<Process> _startProcess(String mainPath, { String packages }) {
   ], environment: <String, String>{ 'FLUTTER_TEST': 'true' });
 }
 
+void _attachStandardStreams(Process process) {
+  for (Stream<List<int>> stream in
+      <Stream<List<int>>>[process.stderr, process.stdout]) {
+    stream.transform(UTF8.decoder)
+      .transform(const LineSplitter())
+      .listen((String line) {
+        if (line != null)
+          print('Shell: $line');
+      });
+  }
+}
+
 class FlutterPlatform extends PlatformPlugin {
   @override
   StreamChannel<String> loadChannel(String mainPath, TestPlatform platform) {
@@ -93,6 +105,8 @@ void main() {
       listenerFile.path, packages: PackageMap.instance.packagesPath
     );
 
+    _attachStandardStreams(process);
+
     void finalize() {
       if (process != null) {
         Process processToKill = process;