Extract out a source_span package from source_maps.

pkg/source_span/lib/src/source_span_exception.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.source_span_exception;
+
+import 'source_span.dart';
+
+/// A class for exceptions that have source span information attached.
+class SourceSpanException implements Exception {
+  /// A message describing the exception.
+  final String message;
+
+  /// The span associated with this exception.
+  ///
+  /// This may be `null` if the source location can't be determined.
+  final SourceSpan span;
+
+  SourceSpanException(this.message, this.span);
+
+  /// Returns a string representation of [this].
+  ///
+  /// [color] may either be a [String], a [bool], or `null`. If it's a string,
+  /// it indicates an ANSII terminal color escape that should be used to
+  /// highlight the span's text. If it's `true`, it indicates that the text
+  /// should be highlighted using the default color. If it's `false` or `null`,
+  /// it indicates that the text shouldn't be highlighted.
+  String toString({color}) {
+    if (span == null) return message;
+    return "Error on " + span.message(message, color: color);
+  }
+}
+
+/// A [SourceSpanException] that's also a [FormatException].
+class SourceSpanFormatException extends SourceSpanException
+    implements FormatException {
+  SourceSpanFormatException(String message, SourceSpan span)
+      : super(message, span);

[Siggi Cherem (dart-lang), 2014/07/16 21:26:10]

there were some changes in the source_maps package on this exception recently,
would you like to copy those changes here too?

[nweiz, 2014/07/17 20:22:08]

Done.

+}