Give Process.kill an optional argument to specify which signal to send.

runtime/bin/process.cc

Index: runtime/bin/process.cc
diff --git a/runtime/bin/process.cc b/runtime/bin/process.cc
index 8c8a78288eb18b0ce8a5d371b2edf110dde0966e..d0682e0e3d73b3082006d79894a986aa238470c2 100644
--- a/runtime/bin/process.cc
+++ b/runtime/bin/process.cc
@@ -147,7 +147,8 @@ void FUNCTION_NAME(Process_Start)(Dart_NativeArguments args) {
 void FUNCTION_NAME(Process_Kill)(Dart_NativeArguments args) {
   Dart_EnterScope();
   intptr_t pid = DartUtils::GetIntegerValue(Dart_GetNativeArgument(args, 1));
-  bool success = Process::Kill(pid);
+  int signal = DartUtils::GetIntegerValue(Dart_GetNativeArgument(args, 2));
+  bool success = Process::Kill(pid, signal);
   Dart_SetReturnValue(args, Dart_NewBoolean(success));
   Dart_ExitScope();
 }