function transformConstructor(constructor, initExpression, superClass) {
  if (superClass) {
    let transformer = new SuperCallTransformer(initExpression);
    return transformer.transformAny(constructor);
  }

  let statements = constructor.body.statements;
  let initStatement = createExpressionStatement(initExpression);
  statements = prependStatements(statements, initStatement);

  return new Method(constructor.location, false,
      constructor.functionKind, constructor.name, constructor.parameterList,
      constructor.typeAnnotation, constructor.annotations,
      createFunctionBody(statements), constructor.debugName);
}
Transforms class constructors when classes have initialized instance
member variables.

When there is a super class, transforms `super()` to
`(super(), initExpression, this)`

Otherwise (no super class), prepend the initExpression to the constructor
body.