Address PR comments

src/libsyntax/json.rs

@@ -200,7 +200,7 @@ fn flush(&mut self) -> io::Result<()> {
             children: db.children.iter().map(|c| {
                 Diagnostic::from_sub_diagnostic(c, je)
             }).chain(sugg).collect(),
-            rendered: Some(output.to_owned()),
+            rendered: Some(output),
         }
     }
 

src/tools/compiletest/src/runtest.rs

@@ -2411,7 +2411,11 @@ fn normalize_output(&self, output: &str, custom_rules: &[(String, String)]) -> S
         let mut normalized = output.replace(&parent_dir_str, "$DIR");
 
         if json {
-            normalized = normalized.replace("\\n", "\n"); // verbatim newline in json strings
+            // escaped newlines in json strings should be readable
+            // in the stderr files. There's no point int being correct,
+            // since only humans process the stderr files.
+            // Thus we just turn escaped newlines back into newlines.
+            normalized = normalized.replace("\\n", "\n");
         }
 
         normalized = normalized.replace("\\\\", "\\") // denormalize for paths on windows