dropping unneeded assertion
dropping unneeded assertion

we don't fall back to broccoli-concat anymore, so we don't need to
enforce its semantics.

joefiorini/broccoli-sourcemap-concat,ember-cli/broccoli-concat,joliss/broccoli-sourcemap-concat,broccolijs/broccoli-concat,rlivsey/broccoli-concat,rlivsey/broccoli-concat,joliss/broccoli-sourcemap-concat,ember-cli/broccoli-concat,ef4/broccoli-sourcemap-concat,ef4/broccoli-sourcemap-concat,joefiorini/broccoli-sourcemap-concat,broccolijs/broccoli-concat