Fix library name (dartanalyzer was complaining in the bots)

lib/src/span_with_context.dart

 // Copyright (c) 2014, 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 source_span.span;
+library source_span.span_with_context;
 
 import 'location.dart';
 import 'span.dart';
 
 /// A class that describes a segment of source text with additional context.
 class SourceSpanWithContext extends SourceSpanBase {
   /// Text around the span, which includes the line containing this span.
   final String context;
 
   /// Creates a new span from [start] to [end] (exclusive) containing [text], in
@@ skipping 13 matching lines @@
           'The context line "$context" must contain "$text".');
     }
 
     var beginningOfLine = context.lastIndexOf('\n', index) + 1;
     if (start.column != index - beginningOfLine) {
       throw new ArgumentError('The span text "$text" must start at '
           'column ${start.column + 1} in a line within "$context".');
     }
   }
 }