Fix incorrect type in override of CompilationError.span.

pkg/front_end/lib/kernel_generator.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.
 
 /// Defines the front-end API for converting source code to Dart Kernel objects.
 library front_end.kernel_generator;
 
 import 'compilation_error.dart';
 import 'compiler_options.dart';
 import 'dart:async';
 
 // TODO(sigmund): move loader logic under front_end/lib/src/kernel/
 import 'package:kernel/analyzer/loader.dart';
 import 'package:kernel/kernel.dart';
+import 'package:source_span/source_span.dart' show SourceSpan;
 
 /// Generates a kernel representation of the program whose main library is in
 /// the given [source].
 ///
 /// Intended for whole program (non-modular) compilation.
 ///
 /// Given the Uri of a file containing a program's `main` method, this function
 /// follows `import`, `export`, and `part` declarations to discover the whole
 /// program, and converts the result to Dart Kernel format.
 ///
@@ skipping 66 matching lines @@
   if (onError == null) return;
   for (var error in errors) {
     onError(new _DartkError(error));
   }
 }
 
 // TODO(sigmund): delete this class. Dartk should not format errors itself, we
 // should just pass them along.
 class _DartkError implements CompilationError {
   String get correction => null;
-  String get span => null;
+  SourceSpan get span => null;
   final String message;
   _DartkError(this.message);
 }