Add a note to Source#install explaining that it should be very light-weight.
Add a note to Source#install explaining that it should be very light-weight.

Review URL: https://chromiumcodereview.appspot.com//10883072

git-svn-id: a962b240025cf0577aa172e20f49ccb159ba6be1@11421 260f80e4-7a28-3924-810f-c04153c831b5

computmaxer/pub,maxwellpeterson-wf/pub,dart-lang/pub,dart-lang/pub,cjkao/dart-pub