Add SourceLocationMixin and SourceLocationBase.

lib/src/location.dart

Index: lib/src/location.dart
diff --git a/lib/src/location.dart b/lib/src/location.dart
index 024c6e2780a340d4eddcc2cefa3094d6c95606b8..afb37c768c97e4018075c094cea1eae25ed1adaf 100644
--- a/lib/src/location.dart
+++ b/lib/src/location.dart
@@ -6,7 +6,13 @@ library source_span.location;
 
 import 'span.dart';
 
-// A class that describes a single location within a source file.
+// TODO(nweiz): Use SourceLocationMixin once we decide to cut a release with
+// breaking changes. See SourceLocationMixin for details.
+
+/// A class that describes a single location within a source file.
+///
+/// This class should not be extended. Instead, [SourceLocationBase] should be
+/// extended instead.
 class SourceLocation implements Comparable<SourceLocation> {
   /// URL of the source containing this location.
   ///
@@ -85,3 +91,10 @@ class SourceLocation implements Comparable<SourceLocation> {
 
   String toString() => '<$runtimeType: $offset $toolString>';
 }
+
+/// A base class for source locations with [offset], [line], and [column] known
+/// at construction time.
+class SourceLocationBase extends SourceLocation {
+  SourceLocationBase(int offset, {sourceUrl, int line, int column})
+      : super(offset, sourceUrl: sourceUrl, line: line, column: column);
+}