@Test
 public void property_get_returns_value() {
   System.getProperties().put("ENV_GET_VAR", "ENV_GET_VALUE");
   SystemFunctions.PropertyGet propertyGet = new SystemFunctions.PropertyGet();
   String propertyVal = (String) propertyGet.apply(ImmutableList.of("ENV_GET_VAR"));
   assertThat("Value should match", propertyVal, equalTo("ENV_GET_VALUE"));
 }
 @Test
 public void property_get_returns_null_if_key_is_not_string() {
   SystemFunctions.PropertyGet propertyGet = new SystemFunctions.PropertyGet();
   String propertyVal = (String) propertyGet.apply(ImmutableList.of(new ArrayList()));
   assertThat("Value should be null", propertyVal, equalTo(null));
 }
 @Test
 public void property_get_nonexistent_returns_null() {
   SystemFunctions.PropertyGet propertyGet = new SystemFunctions.PropertyGet();
   String propertyVal = (String) propertyGet.apply(ImmutableList.of("PROPERTY_MISSING"));
   assertThat("Value should not exist", propertyVal, equalTo(null));
 }