Use the exitCode attribute rather than exit().

lib/src/executable.dart

@@ -33,7 +33,7 @@ void main(List<String> args) {
   if (options['version'] as bool) {
     _loadVersion().then((version) {
       print(version);
-      exit(0);
+      exitCode = 0;
     });
     return;
   }
@@ -42,7 +42,8 @@ void main(List<String> args) {
     print("Compile Sass to CSS.\n");
     print("Usage: dart-sass <input>\n");
     print(argParser.usage);
-    exit(64);
+    exitCode = 64;
+    return;
   }
 
   var color = options['color'] as bool;
@@ -60,7 +61,7 @@ void main(List<String> args) {
 
     // Exit code 65 indicates invalid data per
     // http://www.freebsd.org/cgi/man.cgi?query=sysexits.
-    exit(65);
+    exitCode = 65;
   } catch (error, stackTrace) {
     if (color) stderr.write('\u001b[31m\u001b[1m');
     stderr.write('Unexpected exception:');
@@ -71,6 +72,7 @@ void main(List<String> args) {
     stderr.writeln();
     stderr.write(new Trace.from(stackTrace).terse.toString());
     stderr.flush();
+    exitCode = 255;
   }
 }
 

lib/src/io/interface.dart

@@ -26,5 +26,5 @@ String readFile(String path) => null;
 /// Returns whether a file at [path] exists.
 bool fileExists(String path) => null;
 
-/// Exits the process with the given [exitCode].
-void exit(int exitCode) {}
+/// Gets and sets the exit code that the process will use when it exits.
+int exitCode;

lib/src/io/node.dart

@@ -44,5 +44,8 @@ external _Stderr get _stderr;
 
 final stderr = new Stderr(_stderr);
 
-@JS("process.exit")
-external int exit(int exitCode);
+@JS("process.exitCode")
+external int get exitCode;
+
+@JS("process.exitCode")
+external set exitCode(int code);

lib/src/io/vm.dart

@@ -4,10 +4,10 @@
 
 import 'dart:io' as io;
 
+export 'dart:io' show exitCode;
+
 io.Stdout get stderr => io.stderr;
 
 String readFile(String path) => new io.File(path).readAsStringSync();
 
 bool fileExists(String path) => new io.File(path).existsSync();
-
-void exit(int exitCode) => io.exit(exitCode);