Support multiple occurrences of text in context

lib/src/span_mixin.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_mixin;
 
 import 'dart:math' as math;
 import 'package:path/path.dart' as p;
 
 import 'colors.dart' as colors;
@@ skipping 47 matching lines @@
     buffer.write('line ${line + 1}, column ${column + 1}');
     if (sourceUrl != null) buffer.write(' of ${p.prettyUri(sourceUrl)}');
     buffer.write(': $message');
 
     if (length == 0 && this is! SourceSpanWithContext) return buffer.toString();
     buffer.write("\n");
 
     var textLine;
     if (this is SourceSpanWithContext) {
       var context = (this as SourceSpanWithContext).context;
-      var textIndex = context.indexOf(text.split('\n').first);
-      var lineStart = context.lastIndexOf('\n', textIndex);
-      if (lineStart != -1) {
-        buffer.write(context.substring(0, lineStart + 1));
-        context = context.substring(lineStart + 1);
+      var lineStart = findLineStart(context, text, column);
+      if (lineStart != null && lineStart > 0) {
+        buffer.write(context.substring(0, lineStart));
+        context = context.substring(lineStart);
       }
       var endIndex = context.indexOf('\n');
       textLine = endIndex == -1 ? context : context.substring(0, endIndex + 1);
       column = math.min(column, textLine.length - 1);
     } else {
       textLine = text.split("\n").first;
       column = 0;
     }
 
     var toColumn =
@@ skipping 15 matching lines @@
     return buffer.toString();
   }
 
   bool operator ==(other) => other is SourceSpan &&
       start == other.start && end == other.end;
 
   int get hashCode => start.hashCode + (31 * end.hashCode);
 
   String toString() => '<$runtimeType: from $start to $end "$text">';
 }