Is there something we can do to prevent ourselves from creating a TextSpan with NaN in it? Shouldn't all of these have a valid span coming in?
However, I added an updated assertion and the following tests fail due to invalid length:

• convertFunctionToEs6Class3.ts
• convertFunctionToEs6Class1.ts
• convertFunctionToEs6Class2.ts
• convertFunctionToEs6ClassJsDoc.ts
• convertFunctionToEs6Class-server.ts
• convertFunctionToEs6Class_emptySwitchCase.ts
• convertToEs6Class_emptyCatchClause.ts

I've been using JavaScript for years and never realized NaN was falsy. 🍶

It's done on our side. For example, in getEditsForAction, we literally have the following check

const length = context.endPosition === undefined ? 0 : context.endPosition - context.startPosition;

whereas we don't do the same thing in getAvailableActions. Don't ask me why. Also I'm sorry I didn't fix it first.

OK, I've changed that and tested with Debug.assert(span.length !== NaN && span.length !== undefined);.
EDIT: Tested with !isNaN(length) too...

@DanielRosenwasser Good to go?