Lint on unnecessary {} in string interpolation

[munificent]

If the interpolated expression is just a simple identifier (and the string after the interpolation is not alphanumeric), then the {} are not needed. Given:

"Hi, ${name}!"

The linter should suggest:

"Hi, $name!"

[pq]

Migrated to github: https://github.com/dart-lang/dart_lint/issues/3

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Added Invalid label.