switch to core-js from es6-symbol and es6-shim

Conflicts:
	lib/6to5/polyfill.js
Clarify usage in node env
Merge pull request #298 from Dashed/patch-2

Clarify usage in node env
feat(system-module): rewrite
Merge branch 'core-js' into 2.0.0

Conflicts:
	lib/6to5/polyfill.js
remove user transformers
fix polyfill regenerator runtime path
nicer local exports remapping in default module formatter
add modules remap tests
add amd module name test
add unknown module formatter test
ignore bin tests under instanbul
add api opts.ast === false test
update core-js
make traversal code simpler
fix formatting on react display name object declaration actual test
Merge branch 'feat-system-module-rewrite' of https://github.com/douglasduteil/6to5 into 2.0.0

Conflicts:
	lib/6to5/transformation/modules/system.js
	test/fixtures/transformation/es6-modules-system/exports-from/expected.js
clean up system module formatter
always add additional newline on the last node
better ClassDeclaration contextual transformation