Support multiple occurrences of text in context

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_with_context;
 
 import 'location.dart';
 import 'span.dart';
+import 'utils.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
   /// the given [context].
   ///
   /// [start] and [end] must have the same source URL and [start] must come
   /// before [end]. [text] must have a number of characters equal to the
   /// distance between [start] and [end]. [context] must contain [text], and
   /// [text] should start at `start.column` from the beginning of a line in
   /// [context].
   SourceSpanWithContext(
       SourceLocation start, SourceLocation end, String text, this.context)
       : super(start, end, text) {
-    var index = context.indexOf(text);
-    if (index == -1) {
+    if (!context.contains(text)) {
       throw new ArgumentError(
           'The context line "$context" must contain "$text".');
     }
 
-    var beginningOfLine = context.lastIndexOf('\n', index) + 1;
-    if (start.column != index - beginningOfLine) {
+    if (findLineStart(context, text, start.column) == null) {
       throw new ArgumentError('The span text "$text" must start at '
           'column ${start.column + 1} in a line within "$context".');
     }
   }
 }