If the width and height of the rendered image are not specified
on the Image widget, the nominal resolution is used to scale
the asset so that it occupies the same amount of screen space
as the main asset would have, just with a higher resolution.
That is, if .../my_icon.png is 72px by 72px, then
.../3.0x/my_icon.png should be 216px by 216px;
but they both render into 72px by 72px (in logical pixels),
if width and height are not specified.

Each entry in the asset section of the pubspec.yaml
should correspond to a real file, with the exception of
the main asset entry. If the main asset entry doesn’t correspond
to a real file, then the asset with the lowest resolution
is used as the fallback for devices with device pixel
ratios below that resolution. The entry should still
be included in the pubspec.yaml manifest, however.

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