(Compare what guava-gwt has to do for maven-javadoc-plugin.)
              -->
              <arg>-sourcepath</arg>
              <arg>doesnotexist</arg>
              <!-- https://errorprone.info/docs/installation#maven -->
              <arg>-XDcompilePolicy=simple</arg>

              <!-- https://errorprone.info/docs/installation#maven -->

                   (Compare what guava-gwt has to do for maven-javadoc-plugin.)
              -->
              <arg>-sourcepath</arg>
              <arg>doesnotexist</arg>
              <!-- https://errorprone.info/docs/installation#maven -->
              <arg>-XDcompilePolicy=simple</arg>

              <!-- https://errorprone.info/docs/installation#maven -->

        assertEquals(out.getProperties().get("outputDirectory"), "${env.DOES_NOT_EXIST}");
    }

    @Test
    public void expressionThatEvaluatesToNullReturnsTheLiteralString() throws Exception {
        Map<String, String> modelProperties = new HashMap<>();
        modelProperties.put("outputDirectory", "${DOES_NOT_EXIST}");

        Model model = Model.newBuilder().properties(modelProperties).build();