Update charted to 0.4.8 and roll

packages/source_span/lib/src/span_exception.dart

Index: packages/source_span/lib/src/span_exception.dart
diff --git a/packages/source_span/lib/src/span_exception.dart b/packages/source_span/lib/src/span_exception.dart
index 36f2488329d1331589ab9dc3f1635e7ba81a7cd4..6d3448b6c4f25082882849c76b5b146a943f6d9e 100644
--- a/packages/source_span/lib/src/span_exception.dart
+++ b/packages/source_span/lib/src/span_exception.dart
@@ -2,21 +2,23 @@
 // 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_exception;
-
 import 'span.dart';
 
 /// A class for exceptions that have source span information attached.
 class SourceSpanException implements Exception {
+  // This is a getter so that subclasses can override it.
   /// A message describing the exception.
-  final String message;
+  String get message => _message;
+  final String _message;
 
+  // This is a getter so that subclasses can override it.
   /// The span associated with this exception.
   ///
   /// This may be `null` if the source location can't be determined.
-  final SourceSpan span;
+  SourceSpan get span => _span;
+  final SourceSpan _span;
 
-  SourceSpanException(this.message, this.span);
+  SourceSpanException(this._message, this._span);
 
   /// Returns a string representation of [this].
   ///
@@ -34,10 +36,12 @@ class SourceSpanException implements Exception {
 /// A [SourceSpanException] that's also a [FormatException].
 class SourceSpanFormatException extends SourceSpanException
     implements FormatException {
-  final source;
+  // This is a getter so that subclasses can override it.
+  dynamic get source => _source;
+  final _source;
 
   int get offset => span == null ? null : span.start.offset;
 
-  SourceSpanFormatException(String message, SourceSpan span, [this.source])
+  SourceSpanFormatException(String message, SourceSpan span, [this._source])
       : super(message, span);
 }