Handle variable declarations in inferrer

tests/compiler/dart2js/inference/data/locals.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.
+
+/*element: main:[null]*/
+main() {
+  uninitializedLocal();
+  initializedLocal();
+}
+
+/*element: uninitializedLocal:[null]*/
+uninitializedLocal() {
+  var local;
+  return local;
+}
+
+/*element: initializedLocal:[exact=JSUInt31]*/
+initializedLocal() {
+  var local = 0;
+  return local;
+}