fix #29733, handle variables that completely shadow parameters

tests/language_strong/shadow_parameter_and_local_test.dart

+// Copyright (c) 2017, 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";
+
+// Regression test for https://github.com/dart-lang/sdk/issues/29733 in DDC.
+foo(a) {
+  var a = 123;
+  return a;
+}
+
+main() {
+  Expect.equals(foo(42), 123);
+}