Make observable transform a barback transform.

pkg/codegen/test/refactor_test.dart

+// Copyright (c) 2013, 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 polymer.test.refactor_test;
+
+import 'package:unittest/unittest.dart';
+import 'package:codegen/refactor.dart';
+import 'package:source_maps/span.dart';
+
+main() {
+  var original = "0123456789abcdefghij";
+  var file = new SourceFile.text('', original);
+
+  test('non conflicting, in order edits', () {
+    var txn = new TextEditTransaction(original, file);
+    txn.edit(2, 4, '.');
+    txn.edit(5, 5, '|');
+    txn.edit(6, 6, '-');
+    txn.edit(6, 7, '_');
+    expect((txn.commit()..build('')).text, "01.4|5-_789abcdefghij");
+  });
+
+  test('non conflicting, out of order edits', () {
+    var txn = new TextEditTransaction(original, file);
+    txn.edit(2, 4, '.');
+    txn.edit(5, 5, '|');
+
+    // Regresion test for issue #404: there is no conflict/overlap for edits
+    // that don't remove any of the original code.
+    txn.edit(6, 7, '_');
+    txn.edit(6, 6, '-');
+    expect((txn.commit()..build('')).text, "01.4|5-_789abcdefghij");
+
+  });
+
+  test('non conflicting edits', () {
+    var txn = new TextEditTransaction(original, file);
+    txn.edit(2, 4, '.');
+    txn.edit(3, 3, '-');
+    expect(() => txn.commit(), throwsA(predicate(
+          (e) => e.toString().contains('overlapping edits'))));
+  });
+}