Add SourceLocationMixin and SourceLocationBase.

lib/src/location_mixin.dart

Index: lib/src/location_mixin.dart
diff --git a/lib/src/location_mixin.dart b/lib/src/location_mixin.dart
new file mode 100644
index 0000000000000000000000000000000000000000..5aa0de5ce5e97cb16696677de44ca71f30573981
--- /dev/null
+++ b/lib/src/location_mixin.dart
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+// Copyright (c) 2015, 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.location_mixin;
+
+import 'location.dart';
+import 'span.dart';
+
+// Note: this class duplicates a lot of functionality of [SourceLocation]. This
+// is because in order for SourceLocation to use SourceLocationMixin,
+// SourceLocationMixin couldn't implement SourceLocation. In SourceSpan we
+// handle this by making the class itself non-extensible, but that would be a
+// breaking change for SourceLocation. So until we want to endure the pain of
+// cutting a release with breaking changes, we duplicate the code here.
+
+/// A mixin for easily implementing [SourceLocation].
+abstract class SourceLocationMixin implements SourceLocation {
+  String get toolString {
+    var source = sourceUrl == null ? 'unknown source' : sourceUrl;
+    return '$source:${line + 1}:${column + 1}';
+  }
+
+  int distance(SourceLocation other) {
+    if (sourceUrl != other.sourceUrl) {
+      throw new ArgumentError("Source URLs \"${sourceUrl}\" and "
+          "\"${other.sourceUrl}\" don't match.");
+    }
+    return (offset - other.offset).abs();
+  }
+
+  SourceSpan pointSpan() => new SourceSpan(this, this, "");
+
+  int compareTo(SourceLocation other) {
+    if (sourceUrl != other.sourceUrl) {
+      throw new ArgumentError("Source URLs \"${sourceUrl}\" and "
+          "\"${other.sourceUrl}\" don't match.");
+    }
+    return offset - other.offset;
+  }
+
+  bool operator ==(other) =>
+      other is SourceLocation &&
+      sourceUrl == other.sourceUrl &&
+      offset == other.offset;
+
+  int get hashCode => sourceUrl.hashCode + offset;
+
+  String toString() => '<$runtimeType: $offset $toolString>';
+}
+