Remove usage of deprecated dart:io constants.

lib/test_process.dart

@@ -149,7 +149,7 @@ class TestProcess {
     // If the process is already dead, do nothing.
     if (await _exitCodeOrNull != null) return;
 
-    _process.kill(ProcessSignal.SIGKILL);
+    _process.kill(ProcessSignal.sigkill);
 
     // Log output now rather than waiting for the exitCode callback so that
     // it's visible even if we time out waiting for the process to die.
@@ -217,7 +217,7 @@ class TestProcess {
   ///
   /// If this is called after the process is already dead, it does nothing.
   Future kill() async {
-    _process.kill(ProcessSignal.SIGKILL);
+    _process.kill(ProcessSignal.sigkill);
     await exitCode;
   }
 

pubspec.yaml

@@ -5,7 +5,7 @@ author: Dart Team <misc@dartlang.org>
 homepage: https://github.com/dart-lang/test_process
 
 environment:
-  sdk: '>=2.0.0-dev.17.0 <2.0.0'
+  sdk: '>=2.0.0-dev.55.0 <2.0.0'
 
 dependencies:
   async: ">=1.12.0 <3.0.0"

test/test_process_test.dart

@@ -99,12 +99,12 @@ void main() {
 
   test("signal sends a signal to the process", () async {
     var process = await startDartProcess(r'''
-      ProcessSignal.SIGHUP.watch().listen((_) => print("HUP"));
+      ProcessSignal.sighup.watch().listen((_) => print("HUP"));
       print("ready");
     ''');
 
     await expectLater(process.stdout, emits('ready'));
-    process.signal(ProcessSignal.SIGHUP);
+    process.signal(ProcessSignal.sighup);
     await expectLater(process.stdout, emits('HUP'));
     process.kill();
   }, testOn: "!windows");