lack of "use strict" directive doesn't always warn

[lukeapage]

This code:

// jshint strict: true, globalstrict: true
a = 1;

(or its 2.9.x + equivalent)

// jshint strict: global
a = 1;

both do not warn about missing a "use strict" directive.

I'm guessing only very short code files suffer this problem (or this would be a big problem) but it requires investigation.

Found during investigation of #2663

[nicolo-ribaudo]

Another test case:

// jshint strict: global
a + b;

[nicolo-ribaudo]

JSHint doesn't warn if the statement is a function or a punctuator (a = 2 is = -> JSHint doesn't warn; var a = 2 is var -> JSHint warns)