support older gist formats

[devoncarew]

• support older gist formats (related to Dartpad Behavior Change : Gist File Name Restriction (can load gist only if the file is named "main.dart") #2873 and others)

This restores support for older (mostly unintentional) gists formats; loading from a gist with a single dart file in the gist with any name. We do now nag about the gist - that authors should prefer naming the gist file main.dart.

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[parlough]

Nice! Works well with the scenarios I tried.