code: public FunctionType createFunctionTypeWithNewReturnType(
        FunctionType existingFunctionType, JSType returnType) {
      return FunctionType.builder(this)
          .copyFromOtherFunction(existingFunctionType)
          .withReturnType(returnType)
          .build();
    }
docs: Creates a new function type based on an existing function type but
  with a new return type.
  @param existingFunctionType the existing function type.
  @param returnType the new return type.