Support Webpack v4

[bengourley]

Goals and changeset

Webpack v4 compatibility

The plugin interface changed from v3 to v4. The changelog lists the incompatibilities. Things that are definitely, or likely to need working around are:

• "Plugins should use Compiler.hooks.xxx.tap(, fn) now" - used here and here
• As far as I have tested from updating the interface on the previous point, there are no further changes needed. However, in #6 it was suggested that the structure of stats.chunks has changed from a flat structure to a nested structure. I haven't been able to reproduce this.

Backwards compatibility for v3

Detecting the presence of Compiler.hooks should be sufficient feature detection to know whether to add the plugin via the v3/v4 interface. Unless there is any confirmation of the second point above, the internals of webpack used by plugin plugins does not need to change:

• compilation.getStats().hasErrors() ✅
• compilation.assets[id].existsAt ✅
• stats.chunks ⁇ TBC

Tests

Tests have been adjusted so that they run on a matrix of node/webpack versions. Webpack v4 no longer supports Node v4 so that is excluded from the matrix.

[foxyblocks]

I’ll be digging into this first thing today