Update: Declare ES6 collections (Map, Set, WeakMap, WeakSet) as built-in globals (fixes #1189)
Fix: mergeConfigs ensures the plugins property exists (fixes #1191).
Merge pull request #1190 from mzgol/es6-collections

Update: Declare ES6 collections (Map, Set, WeakMap, WeakSet) as built-in globals (fixes #1189)
Merge pull request #1192 from lo1tuma/fix-merge-configs

Fix: mergeConfigs ensures the plugins property exists (fixes #1191).