[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: https://github.com/dart-lang/sdk/issues/60730
Change-Id: I8c56da29e5cd7c6222a7c5ed7e685501a279030f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/435123
Reviewed-by: Daco Harkes <dacoharkes@google.com>
Commit-Queue: Michael Goderbauer <goderbauer@google.com>

https://dart.googlesource.com/sdk/+/77d75fc73505de1ebeeefc0ee32b2363271ea60b

DEPS

diff --git a/DEPS b/DEPS
index 86d225e..253b8f1 100644
--- a/DEPS
+++ b/DEPS
@@ -235,7 +235,7 @@
 ]
 
 deps = {
-  'flutter/engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + 'a48f47f5446db70fdfdd0ddd053c3dbc8fed7eea',
+  'flutter/engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + '77d75fc73505de1ebeeefc0ee32b2363271ea60b',
 
   'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '2d5987027db4ff25adb9b57c466a5a6c553c4c22',
 

commits.json

diff --git a/commits.json b/commits.json
index 1f1f9bf..2ac7885 100644
--- a/commits.json
+++ b/commits.json
@@ -1,4 +1,4 @@
 {
   "flutter":"2d5987027db4ff25adb9b57c466a5a6c553c4c22",
-  "flutter/engine/src/flutter/third_party/dart":"a48f47f5446db70fdfdd0ddd053c3dbc8fed7eea"
+  "flutter/engine/src/flutter/third_party/dart":"77d75fc73505de1ebeeefc0ee32b2363271ea60b"
 }
\ No newline at end of file