Extract out a source_span package from source_maps.

pkg/source_span/lib/src/span.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.span;
+
+import 'location.dart';
+import 'span_mixin.dart';
+
+/// A class that describes a segment of source text.
+abstract class SourceSpan implements Comparable<SourceSpan> {
+  /// The start location of this span.
+  final SourceLocation start;
+
+  /// The end location of this span, exclusive.
+  final SourceLocation end;
+
+  /// The source text for this span.
+  final String 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;
+
+  /// The length of this span, in characters.
+  final int length;
+
+  /// Creates a new span from [start] to [end] (exclusive) containing [text].
+  ///
+  /// [start] and [end] must have the same source URL and [start] must come
+  /// before [end]. [text] must have a number of characters equal to the
+  /// distance between [start] and [end].
+  factory SourceSpan(SourceLocation start, SourceLocation end, String text) =>
+      new SourceSpanBase(start, end, text);
+
+  /// Creates a new span that's the union of [this] and [other].
+  ///
+  /// The two spans must have the same source URL and may not be disjoint.
+  /// [text] is computed by combining [this.text] and [other.text].
+  SourceSpan union(SourceSpan other);
+
+  /// Compares two spans.
+  ///
+  /// [other] must have the same source URL as [this]. This orders spans by
+  /// [start] then [length].
+  int compareTo(SourceSpan other);
+
+  /// Formats [message] in a human-friendly way associated with this span.
+  ///
+  /// [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 message(String message, {color});
+}
+
+/// 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;
+
+  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.");
+    } else if (end.offset < start.offset) {
+      throw new ArgumentError('End $end must come after start $start.');
+    } else if (text.length != start.distance(end)) {
+      throw new ArgumentError('Text "$text" must be ${start.distance(end)} '
+          'characters long.');
+    }
+  }
+}