Move generated files to gen/.

test/codegen/try_catch.dart

-// 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.
-
-foo() {
-  try {
-    throw "hi there";
-  } on String catch (e, t) {
-  } catch (e, t) {
-    rethrow;
-  }
-}
-
-bar() {
-  try {
-    throw "hi there";
-  } catch (e, t) {
-  } on String catch (e, t) {
-    // unreachable
-    rethrow;
-  }
-}
-
-baz() {
-  try {
-    throw "finally only";
-  } finally {
-    return true;
-  }
-}
-
-qux() {
-  try {
-    throw "on only";
-  } on String catch (e, t) {
-    // unreachable
-    rethrow;
-  }
-}
-
-wub() {
-  try {
-    throw "on without exception parameter";
-  } on String {
-  }
-}
-
-main() {
-  foo();
-  bar();
-  baz();
-  qux();
-  wub();
-}