Factor out some common logic from the launchers.

lib/src/util/dart.dart

 // Copyright (c) 2015, 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.
 
 library test.util.dart;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:isolate';
 
 import 'package:analyzer/analyzer.dart';
 import 'package:path/path.dart' as p;
 import 'package:source_span/source_span.dart';
 
 import 'string_literal_iterator.dart';
 import 'io.dart';
 import 'isolate_wrapper.dart';
-import 'remote_exception.dart';
 
 /// Runs [code] in an isolate.
 ///
 /// [code] should be the contents of a Dart entrypoint. It may contain imports;
 /// they will be resolved in the same context as the host isolate. [message] is
 /// passed to the [main] method of the code being run; the caller is responsible
 /// for using this to establish communication with the isolate.
 ///
 /// [packageRoot] controls the package root of the isolate. It may be either a
 /// [String] or a [Uri].
@@ skipping 71 matching lines @@
   }
 
   var start = contextRunes.offset;
   for (var spanRune in span.text.runes) {
     if (spanRune != contextRunes.current) return null;
     contextRunes.moveNext();
   }
 
   return file.span(start, contextRunes.offset);
 }