[dartdev] Use underscore for output dir name of dart build

Follow-up to https://dart-review.googlesource.com/c/sdk/+/433160.

This aligns us with how `dart --version` and `Abi.current` prints the
OS/architecture pair.

Cq-Include-Trybots: luci.dart.try:pkg-linux-debug-try,pkg-linux-release-arm64-try,pkg-linux-release-try,pkg-mac-release-arm64-try,pkg-win-release-arm64-try,pkg-mac-release-try,pkg-win-release-try
Bug: #60730
Change-Id: I8c56da29e5cd7c6222a7c5ed7e685501a279030f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/435123
Reviewed-by: Daco Harkes <[email protected]>
Commit-Queue: Michael Goderbauer <[email protected]>

Author: goderbauer

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

@@ -65,7 +65,7 @@ bundle/
         Directory.fromUri(Directory.current.uri.resolve('bin/'));
 
     final outputDirectoryDefault = Directory.fromUri(Directory.current.uri
-        .resolve('build/cli/${OS.current}-${Architecture.current}/'));
+        .resolve('build/cli/${OS.current}_${Architecture.current}/'));
     entryPoints = binDirectory.existsSync()
         ? binDirectory
             .listSync()

pkg/dartdev/test/native_assets/build_test.dart

@@ -32,7 +32,7 @@ void main([List<String> args = const []]) async {
 
   /// The relative uri from the package root to the app bundle.
   final relativeBundleUri = Uri.directory(
-      './build/cli/${OS.current}-${Architecture.current}/bundle/');
+      './build/cli/${OS.current}_${Architecture.current}/bundle/');
 
   for (final verbose in [true, false]) {
     final testModifier = verbose ? ' verbose' : '';