This fixes this warning in closure_js_binary rule:
dependency_mode=LOOSE is deprecated and will be removed soon;
prefer to use its equivalent PRUNE_LEGACY
that was added in this PR: [1].
[1] https://github.com/bazelbuild/rules_closure/pull/444
Change-Id: I648f581a8f585402543b97ce7a7ef056ca298c3e