PATCH: Robustify prerequisites

[Mark Mitchell]

Nathaniel found a GUI bug last night, and that sent me down a bit of a
rabbit hole finding other little places where we could be more robust. 
(For example, we would try to create a "<select>" tag with no contained
"<option>" tags, which is not valid HTML.)

I also added a test case for the tuple bug that Nathaniel found last
night.

-- 
Mark Mitchell
CodeSourcery, LLC
mark at codesourcery.com
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