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See eb815be907 for more information about this feature.
The proto modifier indicates that a property declared using `x: T` syntax is actually present on the prototype object of the class, rather than an own property.
The proto and static modifiers are mutually exclusive, as class declarations don't simultaneously define the static prototype object, as they do the instance prototype.
This syntax is only supported on declared classes, not object types, interfaces, or runtime class declarations, and as such should only appear in library definitions.
