Fix new lines in build.dart

Fixes errors of the sort

```
mosum@mosum2:~/projects/i18n/pkgs/intl4x/example$ dart --enable-experiment=record-use build cli example.dart
There are multiple possible targets in the `bin/` directory, and the 'target' argument wasn't specified.mosum@mosum2
```

Change-Id: I7ed6dbd21838506c1fe60b6d69d6066406d3b7c7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/467842
Auto-Submit: Moritz Sümmermann <[email protected]>
Reviewed-by: Brian Quinlan <[email protected]>
Commit-Queue: Brian Quinlan <[email protected]>

Author: mosuem

pkg/dartdev/lib/src/commands/build.dart

@@ -136,14 +136,14 @@ then that is used instead.''',
     // AOT compilation isn't supported on ia32. Currently, generating an
     // executable only supports AOT runtimes, so these commands are disabled.
     if (Platform.version.contains('ia32')) {
-      stderr.write("'dart build' is not supported on x86 architectures.");
+      stderr.writeln("'dart build' is not supported on x86 architectures.");
       return 64;
     }
     final args = argResults!;
 
     var target = args.option('target');
     if (target == null) {
-      stderr.write(
+      stderr.writeln(
         'There are multiple possible targets in the `bin/` directory, '
         "and the 'target' argument wasn't specified.",
       );