infer const wherever required inside a const expression

[DartBot]

This issue was originally filed by [email protected]

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Const expressions have a lot of boilerplate:

    themes: const {
      "Default": const Theme(const {LoginForm: const _LoginForm("pure-form")}),
      "Stacked": const Theme(const {LoginForm: const _LoginForm("pure-form pure-form-stacked")})
    },

It seems like inside any const expression, Dart should infer "const" wherever it can, at least for map and list literals, and ideally for objects as well.

If I leave out "const" for the map literals, the error is:

"Arguments of a constant creation must be constant expressions".

[DartBot]

This comment was originally written by @seaneagan

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dupe of issue #4046 / issue #18241

[sethladd]

Added Area-Language, Triaged labels.

[lrhn]

Added Duplicate label.
Marked as being merged into #4046.