fix(lint): enforce that module-private members have @internal.

This is needed to prevent leaking internal APIs to users via our published .d.ts typings.
Fixes #4645

Closes #4989

gulpfile.js

@@ -294,6 +294,7 @@ gulp.task('lint', ['build.tools'], function() {
   // https://github.com/palantir/tslint#supported-rules
   var tslintConfig = {
     "rules": {
+      "requireInternalWithUnderscore": true,
       "requireParameterType": true,
       "requireReturnType": true,
       "semicolon": true,