repl: don’t write to input stream in editor mode

Instead of writing to the REPL’s input stream for the alignment
spaces in `.editor` mode, let `readline` handle the spaces
properly (echoing them using `_ttyWrite` and adding them to the
current line buffer).

Fixes: nodejs#9189

lib/repl.js

@@ -476,7 +476,7 @@ function REPLServer(prompt,
       const matches = self._sawKeyPress ? cmd.match(/^\s+/) : null;
       if (matches) {
         const prefix = matches[0];
-        self.inputStream.write(prefix);
+        self.write(prefix);
         self.line = prefix;
         self.cursor = prefix.length;
       }

test/parallel/test-repl-.editor.js

@@ -75,12 +75,15 @@ tests.forEach(run);
 // Auto code alignment for .editor mode
 function testCodeAligment({input, cursor = 0, line = ''}) {
   const stream = new common.ArrayStream();
+  const outputStream = new common.ArrayStream();
+
+  stream.write = () => { throw new Error('Writing not allowed!'); };
 
   const replServer = repl.start({
     prompt: '> ',
     terminal: true,
     input: stream,
-    output: stream,
+    output: outputStream,
     useColors: false
   });