diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6bd18f1..ab13bda 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,9 @@
 # 1.2.0
 
+* **Deprecated:** Extending `SourceLocation` directly is deprecated. Instead,
+  extend the new `SourceLocationBase` class or mix in the new
+  `SourceLocationMixin` mixin.
+
 * Dramatically improve the performance of `FileLocation`.
 
 # 1.1.6
diff --git a/lib/source_span.dart b/lib/source_span.dart
index 89b1650..9666dc2 100644
--- a/lib/source_span.dart
+++ b/lib/source_span.dart
@@ -6,6 +6,7 @@
 
 export "src/file.dart";
 export "src/location.dart";
+export "src/location_mixin.dart";
 export "src/span.dart";
 export "src/span_exception.dart";
 export "src/span_mixin.dart";
diff --git a/lib/src/file.dart b/lib/src/file.dart
index c2b97e7..95fa92c 100644
--- a/lib/src/file.dart
+++ b/lib/src/file.dart
@@ -8,6 +8,7 @@
 import 'dart:typed_data';
 
 import 'location.dart';
+import 'location_mixin.dart';
 import 'span.dart';
 import 'span_mixin.dart';
 import 'span_with_context.dart';
@@ -212,17 +213,19 @@
 /// and column values based on its offset and the contents of [file].
 ///
 /// A [FileLocation] can be created using [SourceFile.location].
-class FileLocation extends SourceLocation {
+class FileLocation extends SourceLocationMixin implements SourceLocation {
   /// The [file] that [this] belongs to.
   final SourceFile file;
 
+  final int offset;
   Uri get sourceUrl => file.url;
   int get line => file.getLine(offset);
   int get column => file.getColumn(offset);
 
-  FileLocation._(this.file, int offset)
-      : super(offset) {
-    if (offset > file.length) {
+  FileLocation._(this.file, this.offset) {
+    if (offset < 0) {
+      throw new RangeError("Offset may not be negative, was $offset.");
+    } else if (offset > file.length) {
       throw new RangeError("Offset $offset must not be greater than the number "
           "of characters in the file, ${file.length}.");
     }
diff --git a/lib/src/location.dart b/lib/src/location.dart
index 024c6e2..afb37c7 100644
--- a/lib/src/location.dart
+++ b/lib/src/location.dart
@@ -6,7 +6,13 @@
 
 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 @@
 
   String toString() => '<$runtimeType: $offset $toolString>';
 }
+
+/// A base class for source locations with [offset], [line], and [column] known
+/// at construction time.
+class SourceLocationBase extends SourceLocation {
+  SourceLocationBase(int offset, {sourceUrl, int line, int column})
+      : super(offset, sourceUrl: sourceUrl, line: line, column: column);
+}
diff --git a/lib/src/location_mixin.dart b/lib/src/location_mixin.dart
new file mode 100644
index 0000000..5aa0de5
--- /dev/null
+++ b/lib/src/location_mixin.dart
@@ -0,0 +1,51 @@
+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library source_span.location_mixin;
+
+import 'location.dart';
+import 'span.dart';
+
+// Note: this class duplicates a lot of functionality of [SourceLocation]. This
+// is because in order for SourceLocation to use SourceLocationMixin,
+// SourceLocationMixin couldn't implement SourceLocation. In SourceSpan we
+// handle this by making the class itself non-extensible, but that would be a
+// breaking change for SourceLocation. So until we want to endure the pain of
+// cutting a release with breaking changes, we duplicate the code here.
+
+/// A mixin for easily implementing [SourceLocation].
+abstract class SourceLocationMixin implements SourceLocation {
+  String get toolString {
+    var source = sourceUrl == null ? 'unknown source' : sourceUrl;
+    return '$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>';
+}
+
diff --git a/pubspec.yaml b/pubspec.yaml
index 7ffbc64..25de799 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: source_span
-version: 1.2.0-dev
+version: 1.2.0
 author: Dart Team <misc@dartlang.org>
 description: A library for identifying source spans and locations.
 homepage: https://github.com/dart-lang/source_span