--print-slow-trees not working

[davewasmer]

As reported by @seawatts in denali-js/core#374:

denali build --print-slow-trees=true
cli v0.0.12 [local] | denali v0.0.29 [local]

✖ Build failed
TypeError: broccoli_slow_trees_1.default is not a function
    at Project.finishBuild (/Users/daw/projects/oss/denali/denali-cli/lib/project.ts:320:21)
    at Project.<anonymous> (/Users/daw/projects/oss/denali/denali-cli/lib/project.ts:193:12)
    at Generator.next (<anonymous>)
    at fulfilled (./node_modules/tslib/tslib.js:93:62)
    at <anonymous>
    at process._tickDomainCallback (internal/process/next_tick.js:228:7)