Avoid uninitialized final fields. (#22)
diff --git a/lib/src/span.dart b/lib/src/span.dart
index 599d668..ff5c4b0 100644
--- a/lib/src/span.dart
+++ b/lib/src/span.dart
@@ -8,22 +8,22 @@
/// A class that describes a segment of source text.
abstract class SourceSpan implements Comparable<SourceSpan> {
/// The start location of this span.
- final SourceLocation start;
+ SourceLocation get start;
/// The end location of this span, exclusive.
- final SourceLocation end;
+ SourceLocation get end;
/// The source text for this span.
- final String text;
+ String get text;
/// The URL of the source (typically a file) of this span.
///
/// This may be null, indicating that the source URL is unknown or
/// unavailable.
- final Uri sourceUrl;
+ Uri get sourceUrl;
/// The length of this span, in characters.
- final int length;
+ int get length;
/// Creates a new span from [start] to [end] (exclusive) containing [text].
///