When running browser tests, e.g. via `gulp unittest`, the test files are not
processed by babel, and neither by the "unittestcli" gulp target.
This commit copies the babelPluginReplaceNonWebPackRequire plugin from the
unittestcli target to the SystemJS config so that `__non_webpack_require__` is
replaced with `require` for all build targets, and adds a unit test to ensure
that this indeed works as expected.
d253889d97