Revert r27592, some unit tests fail.

tests/language/assertion2_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.
-
-import "package:expect/expect.dart";
-import "compiler_annotations.dart";
-
-@DontInline()
-foo() {
-  () => 42;
-  int a;
-  assert((a = 2) == 2);
-  return a;
-}
-
-main() {
-  bool isAssertEnabled = false;
-  assert(isAssertEnabled = true);
-  if (isAssertEnabled) {
-    Expect.equals(44, foo() + 42);
-  } else {
-    Expect.throws(() => foo() + 42, (e) => e is NoSuchMethodError);
-  }
-}