Remove unsafe flag from ddc sdk

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'
-  ];
+  var args = ['--no-source-map', '--no-emit-metadata'];
   args.addAll(arguments);
   args.addAll([
     'dart:_runtime',
     'dart:_debugger',
     'dart:_foreign_helper',
     'dart:_interceptors',
     'dart:_internal',
     'dart:_isolate_helper',
     'dart:_js_embedded_names',
     'dart:_js_helper',
@@ skipping 18 matching lines @@
     'dart:html_common',
     'dart:svg',
     'dart:web_audio',
     'dart:web_gl',
     'dart:web_sql'
   ]);
 
   var result = compile(args);
   exit(result);
 }