Add exitCode getter counterpart to the existing setter in dart:io.

runtime/bin/process.cc

Index: runtime/bin/process.cc
diff --git a/runtime/bin/process.cc b/runtime/bin/process.cc
index 09476b9100971e4adbc677f55bdaee3d9244950f..5d392d23fffcb6389810e40d42833359a1214623 100644
--- a/runtime/bin/process.cc
+++ b/runtime/bin/process.cc
@@ -221,6 +221,11 @@ void FUNCTION_NAME(Process_SetExitCode)(Dart_NativeArguments args) {
 }
 
 
+void FUNCTION_NAME(Process_GetExitCode)(Dart_NativeArguments args) {
+  Dart_SetReturnValue(args, Dart_NewInteger(Process::GlobalExitCode()));
+}
+
+
 void FUNCTION_NAME(Process_Sleep)(Dart_NativeArguments args) {
   int64_t milliseconds = 0;
   // Ignore result if passing invalid argument and just set exit code to 0.