target=e6 with module=commonjs need "use strict" in the output

[3n-mb]

tsc -v 1.7.5
When compiling for es6 target, with let, const, etc., and using commonjs module, browser (Chrome) complains about let and co. used outside of "strict mode" zone. Adding "use strict" at the beginning of commonjs module should fix this.

Note:

• spec for es6, 10.2.1 Strict Mode Code says that "Module code is always strict mode code". This assumes that es6 modules are used, while commonjs' require()'s look differently.
• it does not seem to be a problem with node, as it probably relaxes this rule.
• Babel's transform-es2015-modules-commonjs inserts "use strict" at the start of the file, satisfying browser's desire to stick to the standard's rule, and it all just works. Unfortunately, this requires a babel step in the build :(

[DanielRosenwasser]

Hey @3n-mb, this seems to be a duplicate of #3576. You can give our nightlies a try by installing npm install -g typescript@next and seeing the new behavior there.