🤯

(I'd call this RichEngineLineMetrics, but that might be my Java subconscious speaking)

Done!

nit: I have never been able to build a clean conceptual model for data classes that used inheritance. For example, the runtimeType check breaks the substitutability property that's expected from the inheriting class. Could composition work here? For example, the EngineLineMetrics could have a nullable field RichMetrics:

class EngineLineMetrics implements ui.LineMetrics {
  const EngineLineMetrics(this.otherProperties) : richMetrics = null;

  const EngineLineMetrics.rich(this.otherProperties, this.richMetrics);

  final RichMetrics? richMetrics;
}

The relationship can also be inverted, such that RichMetrics contains EngineLineMetrics. Depends on the use-case.

Composition should still be able to clean up most of the noise with unrelated properties, but it would avoid complexities from inheritance, such as the unusual usage of super in hashCode and operator ==, and the shaky substitutability of the two types.

The sample you provided could work, but it means I have to add ! everywhere:

lineMetrics.richMetrics!.*

Reversing the composition might work better in this case:

class ParagraphLine { // Instead of RichEngineLineMetrics
  const ParagraphLine(this.lineMetrics, ... other properties);

  final ui.LineMetrics lineMetrics;
}