[VM Service] Fix Windows hot reload timeout when using experimental service

Forward the `pause` parameter to the C++ runtime in reloadSources, and use
the path resolver helper to properly resolve raw Windows file paths during
hot reload compilation.

TAG=agy
CONV=ae3a29f5-e2c8-499c-b221-ff61e40f5f1f

Change-Id: Iab29cb94b390144d17f1c014ec2e9acd1ab1e11e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/505160
Commit-Queue: Ben Konyi <bkonyi@google.com>
Auto-Submit: Ben Konyi <bkonyi@google.com>
Reviewed-by: Nicholas Shahan <nshahan@google.com>

pkg/dart_runtime_service_vm/lib/src/dart_runtime_service_vm_rpcs.dart

@@ -37,6 +37,7 @@ final class DartRuntimeServiceVmRpcs {
   static const _kUserTags = 'userTags';
   static const _kRootLibUri = 'rootLibUri';
   static const _kForce = 'force';
+  static const _kPause = 'pause';
   static const _kKernelFilePath = 'kernelFilePath';
 
   late final rpcs = UnmodifiableListView<ServiceRpcHandler>([
@@ -154,7 +155,7 @@ final class DartRuntimeServiceVmRpcs {
       final outputDill = tempDir.childFile('for_hot_reload.dill');
       try {
         await frontend_server.invokeCompile(
-          executable: Uri.parse(rootLibUri).toFilePath(),
+          executable: _maybeUriToFilename(rootLibUri),
           outputDill: outputDill.path,
           serverInfoFile: residentCompilerInfoFile,
         );
@@ -171,6 +172,7 @@ final class DartRuntimeServiceVmRpcs {
           _kIsolateId: isolateId,
           _kKernelFilePath: outputDill.uri.toFilePath(),
           _kForce: parameters[_kForce].asBoolOr(false),
+          _kPause: parameters[_kPause].asBoolOr(false),
         },
       );
     } finally {
@@ -178,3 +180,18 @@ final class DartRuntimeServiceVmRpcs {
     }
   }
 }
+
+/// Safely converts [maybeUri] to a local file path.
+///
+/// Returns [maybeUri] as-is if it is already a raw path (e.g. `C:\foo.dart`
+/// or `/foo.dart`) or a non-file URI. This avoids crashes on Windows where
+/// drive letters (like `C:`) are mistaken for URI schemes by [Uri.parse].
+String _maybeUriToFilename(String maybeUri) {
+  final uri = Uri.tryParse(maybeUri);
+  if (uri != null && (uri.scheme == 'file' || uri.scheme == '')) {
+    try {
+      return uri.toFilePath();
+    } catch (_) {}
+  }
+  return maybeUri;
+}