[strong] Adding regression cases which should always continue working.

test/mjsunit/strong/use-before-var.js

+// Copyright 2015 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --strong-mode
+
+function strongFunction() {
+  "use strong";
+  // This should not be a "use before declaration" error, even though the
+  // variable is defined later (by non-strong code). Rationale: Non-strong code
+  // is allowed to use var declarations, and the semantics of var declarations
+  // say that the variable should be usable before the declaration.
+  return notStrong + 123;
+}
+
+var notStrong = 456;
+
+(function TestStrongFunctionUsingLaterDefinedNonStrongVar() {
+  assertEquals(strongFunction(), 579);
+})();