Provide an editor-centric option for mac & linux

Intended for A/B testing.

I was thinking of serving them up 50-50, so we can see how people
react to each version.

NOTE: The code for these pages is somewhat complicated and tangled—lots
of includes. I'd appreciate at least one of the reviewers doing a very careful
review of the Windows, Linux, and Mac versions of each page. (It'd be nice
to get a careful review of the includes, too, but that's less important than the
end result.)

Staged at:
https://downloads-dot-dart-lang.appspot.com/tools/download.html
(more or less what's at https://www.dartlang.org/tools/download.html today)

https://downloads-dot-dart-lang.appspot.com/tools/download-editor.html
(emphasizes downloading Dart Editor; the Windows version doesn't change,
but the Mac & Linux ones do)

BUG= https://code.google.com/p/dart/issues/detail?id=21857
R=sethladd@google.com

Review URL: https://chromiumcodereview.appspot.com/789363003