But don't test webpack 4 on old node versions since it now requires 6 or
later.

webpack 4 drops the 'loaders' key on the config

webpack has a completely new plugin interface. See changes to
wepack/tapable for in-depth.

In the future the file-loader should move away from loadContext.options
and this would not be needed.

Webpack 4 now requires either 'production', 'development', or 'none'.

The new method with AsyncSeriesWaterfallHook already merges the initial
value with the result so there's no need to worry about the result not
being defined.

Recompiling and caching only really makes sense for development builds.

toposort is non-deterministic when there's a tie. Needed to allow for
either result in the test.

This happens when A and B both depend on C, dependency graph looks like:

      A
     /
    C
     \
      B

Ideally we could pass a tie breaker sort function to toposort to fix
this.

Previously was just doing a strait string compare which was very hard to
tell what the differences causing the failing were.

minification is now on by default for production mode.

webpack-4.0.0-alpha.5 introduced a large refactor to chunks. This is a
corresponding update to handle the new structure.

This change introduces an API change to `chunksorter.dependency`. It now
requires an additional parameter `chunkGroups` that is available from
the compilation object.

To read more about the CommonsChunkPlugin changes see:
- webpack/webpack#6357
- https://gist.github.com/sokra/1522d586b8e5c0f5072d7565c2bee693

Tests were green before this so can have higher confidence in this
change.

Wanted to update the test fixtures to not use minification, but needed
to go back to a stable state to do that.

Appear to all be internal webpack changes.

It is a bit painful to review. Certain parts may be easier to see the
changes by using the --word-diff-regex flag. Example:

    git diff --word-diff-regex=[^[:space:]\"\)]+

Need a webpack 4 compatible version.

Perf improvement.

Using getParents() causes a full iteration over the parents Set to get
an array. Then using map() is a second iteration over the resulting
array. By using Array.from(iterable, mapFn) we only iterate once over
the internal data structure of parents.