fix(server): throw if multiple instances use the same compiler - #3039

fix(server): throw if multiple instances use the same compiler#3039

ylemkimon wants to merge 4 commits into

webpack:masterwebpack/webpack-dev-server:masterfrom

ylemkimon:dev-server-propertyylemkimon/webpack-dev-server:dev-server-propertyCopy head branch name to clipboard