Add abstract references base #205
add spec references
Merge branch 'master' into abstract-references

Conflicts:
	lib/6to5/polyfill.js
Merge branch 'master' into abstract-references
complete abstract references support - parser support left
add abstract reference CallExpression base
upgrade acorn-6to5
remove SequenceExpression user whitespace
add support for call expression abstract references and more versatile tests
add WeakMap existence check to symbol polyfilling
Merge branch 'abstract-references'