Add (failing) source_update_test.dart.

dart/tests/try/source_update_test.dart

+// 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.
+
+// SharedOptions=--package-root=sdk/lib/_internal/
+
+import 'test_try.dart';
+
+InteractionContext interaction;
+
+void main() {
+  interaction = mockTryDartInteraction();
+
+  runTests(<TestCase>[
+
+    new TestCase('Test setting full source', () {
+      clearEditorPaneWithoutNotifications();
+      mainEditorPane.appendText('Foo\nBar');
+    }, () {
+      expectSource('Foo\nBar');
+    }),
+
+    new TestCase('Test modifying a single line', () {
+      Element lastLine = mainEditorPane.lastChild;
+      lastLine.appendText('Baz');
+    }, () {
+      expectSource('Foo\nBarBaz');
+    }),
+
+  ]);
+}
+
+void expectSource(String expected) {
+  String actualSource = interaction.currentCompilationUnit.content;
+  Expect.stringEquals(expected, actualSource);
+}