PatchScreen added a reference to itself to the shared copy of the
ListenableAccountDiffPreference for a change. Every file contained
by that PatchSet uses the same ListenableAccountDiffPreference,
so the PatchScreen and all of its DOM elements and widgets and even
the last cached PatchScript leaked across file navigation.
Bug: issue 1370
Change-Id: Iae588a34878f83d95bd1b03913d3a8523afdf58c
92817e8784