Make StringScannerException extend SpanFormatException.

pkg/string_scanner/lib/src/exception.dart

Index: pkg/string_scanner/lib/src/exception.dart
diff --git a/pkg/string_scanner/lib/src/exception.dart b/pkg/string_scanner/lib/src/exception.dart
index c3986871a8cff41e0f5219616c6700faa7ac8964..44ec9a3e715720b4e3d646e41330049eec8241f4 100644
--- a/pkg/string_scanner/lib/src/exception.dart
+++ b/pkg/string_scanner/lib/src/exception.dart
@@ -7,10 +7,7 @@ library string_scanner.exception;
 import 'package:source_maps/source_maps.dart';
 
 /// An exception thrown by a [StringScanner] that failed to parse a string.
-class StringScannerException implements FormatException {
-  /// The error message.
-  final String message;
-
+class StringScannerException extends SpanFormatException {
   /// The source string being parsed.
   final String string;
 
@@ -19,19 +16,6 @@ class StringScannerException implements FormatException {
   /// This may be `null`, indicating that the source URL is unknown.
   final Uri sourceUrl;
 
-  /// The span within [string] that caused the exception.
-  final Span span;
-
-  StringScannerException(this.message, this.string, this.sourceUrl, this.span);
-
-  /// Returns a detailed description of this exception.
-  ///
-  /// If [useColors] is true, the section of the source that caused the
-  /// exception will be colored using ANSI color codes. By default, it's colored
-  /// red, but a different ANSI code may passed via [color].
-  String toString({bool useColors: false, String color}) {
-    return "Error on " + span.getLocationMessage(
-        message, useColors: useColors, color: color);
-  }
+  StringScannerException(String message, this.string, this.sourceUrl, Span span)
+      : super(message, span);
 }
-