Cleanup build_sdk error checking

pkg/dev_compiler/tool/build_sdk.dart

 #!/usr/bin/env dart
 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// A straightforward script that builds the SDK.
 ///
 /// This would be easy enough to do in a shell script, as we go through the
 /// command line interface. But being able to build from a Dart library means
 /// we can call this during code coverage to get more realistic numbers.
 
+import 'dart:io';
+
 import 'package:dev_compiler/src/compiler/command.dart';
 
 main(List<String> arguments) {
   var args = [
     '--unsafe-force-compile',
     '--no-source-map',
     '--no-emit-metadata'
   ];
   args.addAll(arguments);
   args.addAll([
@@ skipping 23 matching lines @@
     'dart:typed_data',
     'dart:indexed_db',
     'dart:html',
     'dart:html_common',
     'dart:svg',
     'dart:web_audio',
     'dart:web_gl',
     'dart:web_sql'
   ]);
 
-  if (compile(args) != 0) {
-    throw 'SDK build failed.';
-  }
+  var result = compile(args);
+  exit(result);
 }