Extract out a source_span package from source_maps.

pkg/source_span/lib/src/span_exception.dart

Index: pkg/source_span/lib/src/span_exception.dart
diff --git a/pkg/source_span/lib/src/span_exception.dart b/pkg/source_span/lib/src/span_exception.dart
new file mode 100644
index 0000000000000000000000000000000000000000..af642413bdda0c2a1a6c20c459b40bd4e4334a5f
--- /dev/null
+++ b/pkg/source_span/lib/src/span_exception.dart
@@ -0,0 +1,43 @@
+// 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.span_exception;
+
+import '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 {
+  final source;
+
+  int get position => span == null ? null : span.start.offset;
+
+  SourceSpanFormatException(String message, SourceSpan span, [this.source])
+      : super(message, span);
+}