[beta] [ Service ] Include drive letter in path when launching DDS snapshot

The previous logic for building the path to dds.dart.snapshot would result
in the Windows drive letter being dropped from the path:

\path\to\dart-sdk\bin\dds.dart.snapshot

This works most of the time since a leading slash is treated as a reference
to the current drive, which often contains the Dart SDK. However, if the SDK
is on a different drive than the current (e.g., in a container with two drives),
the VM will fail to find the snapshot.

This change uses the File(...) APIs from dart:io to build the path rather than
trying to use the Uri class to manually hack together a path.

TEST=N/A, not reproducible without a second Windows drive

Bug: https://github.com/grpc/grpc-dart/issues/697
Change-Id: I71d00b07a98508a780f5aab76417da4aa530f3c4
Cherry-pick: https://dart-review.googlesource.com/c/sdk/+/360920
Cherry-pick-request: https://github.com/dart-lang/sdk/issues/55386
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/361400
Reviewed-by: Siva Annamalai <asiva@google.com>

sdk/lib/_internal/vm/bin/vmservice_io.dart

@@ -96,25 +96,15 @@ class _DebuggingSession {
     bool disableServiceAuthCodes,
     bool enableDevTools,
   ) async {
-    final dartPath = Uri.parse(Platform.resolvedExecutable);
-    final dartDir = [
-      '', // Include leading '/'
-      ...dartPath.pathSegments.sublist(
-        0,
-        dartPath.pathSegments.length - 1,
-      ),
-    ].join('/');
-
+    final dartDir = File(Platform.resolvedExecutable).parent.path;
     final fullSdk = dartDir.endsWith('bin');
     final snapshotName = [
       dartDir,
       fullSdk ? 'snapshots' : 'gen',
       'dds.dart.snapshot',
-    ].join('/');
-    final execName = dartPath.toString();
-
+    ].join(Platform.pathSeparator);
     _process = await Process.start(
-      execName,
+      Platform.resolvedExecutable,
       [
         snapshotName,
         '--vm-service-uri=${server!.serverAddress!}',