Extract out a source_span package from source_maps.

pkg/source_span/lib/src/span_mixin.dart

Index: pkg/source_span/lib/src/span_mixin.dart
diff --git a/pkg/source_span/lib/src/span_mixin.dart b/pkg/source_span/lib/src/span_mixin.dart
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+++ b/pkg/source_span/lib/src/span_mixin.dart
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+// 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_mixin;
+
+import 'package:path/path.dart' as p;
+
+import 'colors.dart' as colors;
+import 'span.dart';
+import 'utils.dart';
+
+/// A mixin for easily implementing [SourceSpan].
+///
+/// This implements the [SourceSpan] methods in terms of [start], [end], and
+/// [text]. This assumes that [start] and [end] have the same source URL, that
+/// [start] comes before [end], and that [text] has a number of characters equal
+/// to the distance between [start] and [end].
+abstract class SourceSpanMixin implements SourceSpan {
+  Uri get sourceUrl => start.sourceUrl;
+  int get length => end.offset - start.offset;
+
+  int compareTo(SourceSpan other) {
+    int d = start.compareTo(other.start);
+    return d == 0 ? end.compareTo(other.end) : d;
+  }
+
+  SourceSpan union(SourceSpan other) {
+    if (sourceUrl != other.sourceUrl) {
+      throw new ArgumentError("Source URLs \"${sourceUrl}\" and "
+          " \"${other.sourceUrl}\" don't match.");
+    }
+
+    var start = min(this.start, other.start);
+    var end = max(this.end, other.end);
+    var beginSpan = start == this.start ? this : other;
+    var endSpan = end == this.end ? this : other;
+
+    if (beginSpan.end.compareTo(endSpan.start) < 0) {
+      throw new ArgumentError("Spans $this and $other are disjoint.");
+    }
+
+    var text = beginSpan.text +
+        endSpan.text.substring(beginSpan.end.distance(endSpan.start));
+    return new SourceSpan(start, end, text);
+  }
+
+  String message(String message, {color}) {
+    if (color == true) color = colors.RED;
+    if (color == false) color = null;
+
+    var buffer = new StringBuffer();
+    buffer.write('line ${start.line + 1}, column ${start.column + 1}');
+    if (sourceUrl != null) buffer.write(' of ${p.prettyUri(sourceUrl)}');
+    buffer.write(': $message');
+    if (length == 0) return buffer.toString();
+
+    buffer.write("\n");
+    var textLine = text.split("\n").first;
+    if (color != null) buffer.write(color);
+    buffer.write(textLine);
+    buffer.write("\n");
+    buffer.write('^' * textLine.length);
+    if (color != null) buffer.write(colors.NONE);
+    return buffer.toString();
+  }
+
+  bool operator ==(SourceSpan other) =>
+      start == other.start && end == other.end;
+
+  int get hashCode => start.hashCode + (31 * end.hashCode);
+
+  String toString() => '<$runtimeType: from $start to $end "$text">';
+}