don't use internal throwOnWarnings option

pkg/front_end/lib/src/base/processed_options.dart

 // Copyright (c) 2017, 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.
 
 import 'dart:async';
 
 import 'package:front_end/compilation_message.dart';
 import 'package:front_end/compiler_options.dart';
 import 'package:front_end/file_system.dart';
 import 'package:front_end/src/base/performace_logger.dart';
@@ skipping 595 matching lines @@
   final Severity severity;
 
   String get message => _original.message;
 
   String get tip => _original.tip;
 
   String get analyzerCode => _original.code.analyzerCode;
 
   String get dart2jsCode => _original.code.dart2jsCode;
 
-  SourceSpan get span =>
-      new SourceLocation(_original.charOffset, sourceUrl: _original.uri)
+  SourceSpan get span => _original.charOffset == -1
+      ? null

[Siggi Cherem (dart-lang), 2017/09/01 20:34:34]

If _original.uri is not null, maybe use SourceLocation(0, sourceUrl:
_original.uri) instead?

+      : new SourceLocation(_original.charOffset, sourceUrl: _original.uri)
           .pointSpan();
 
   _CompilationMessage(this._original, this.severity);
 
   String toString() => message;
 }