Function-level 'use strict' is not bug-compatible

[gbrail]

Some existing code that worked on previous versions of Rhino fails in 1.7.7.2. For instance:

function foo() {
'use strict';
bar = 1234;
}

This would work previously because Rhino used to ignore "use strict" inside a function.

We should change this enhancement to work only when we have the language level set to "ES6" or above.

[gbrail]

Addressed in #361.

[gbrail]

This feature now only works if the language level is set to "ES6" or higher.