Wrapper for strings that conform to a data type or language.
Implementations of this interface are wrappers for strings, and typically
associate a type contract with the wrapped string. Concrete implementations
of this interface may choose to implement additional run-time type checking,
see for example goog.html.SafeHtml. If available, client code that
needs to ensure type membership of an object should use the type's function
to assert type membership, such as goog.html.SafeHtml.unwrap.