Move dart2js from sdk/lib/_internal/compiler to pkg/compiler

BUG=

Review URL: https://codereview.chromium.org//690103004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41512 260f80e4-7a28-3924-810f-c04153c831b5
Revert "Move dart2js from sdk/lib/_internal/compiler to pkg/compiler"

This reverts commit r41512.

BUG=

Review URL: https://codereview.chromium.org//693183006

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41513 260f80e4-7a28-3924-810f-c04153c831b5