Allow some fields to be overridden in strong mode.

lib/src/span.dart

Index: lib/src/span.dart
diff --git a/lib/src/span.dart b/lib/src/span.dart
index fe1ac3956bcbcc4f0dd1e0ca492959424de60e85..6ee68b494de361d2e421c1a483f62a46523ef4ba 100644
--- a/lib/src/span.dart
+++ b/lib/src/span.dart
@@ -58,11 +58,18 @@ abstract class SourceSpan implements Comparable<SourceSpan> {
 /// A base class for source spans with [start], [end], and [text] known at
 /// construction time.
 class SourceSpanBase extends SourceSpanMixin {
-  final SourceLocation start;
-  final SourceLocation end;
-  final String text;
+  // These fields go through getters so that subclasses can override them.
+
+  SourceLocation get start => _start;
+  final SourceLocation _start;
+
+  SourceLocation get end => _end;
+  final SourceLocation _end;
+
+  String get text => _text;
+  final String _text;
 
-  SourceSpanBase(this.start, this.end, this.text) {
+  SourceSpanBase(this._start, this._end, this._text) {
     if (end.sourceUrl != start.sourceUrl) {
       throw new ArgumentError("Source URLs \"${start.sourceUrl}\" and "
           " \"${end.sourceUrl}\" don't match.");