Add "use strict" directive (#7411)

* #7349
- Add "use strict" directive to transformed class bodies

* Update tests (#7349)

* Add test for program with use strict directive (#7349)

* Update tests (#7349)

* Update source mapping (#7349)

* Add test for parent blockStatement with use strict directive (#7349)

* Update tests (#7349)

* Update tests (#7349)

* Update tests (#7349)

* Add test for constructor-only class (#7349)

* Constructor only classes are strict, too

But constructor only that use non-simple parameters must use a strict function wrapper.

* Fix test

* Wrapper not needed if class is already strict

* Revert change to lerna.json

packages/babel-plugin-transform-classes/test/fixtures/loose-classCallCheck/class/output.js

@@ -1 +1,3 @@
-let A = function A() {};
+let A = function A() {
+  "use strict";
+};