and DoesNotExist.
                            type: string
                          values:
                            description: values is an array of string values. If the
                              operator is In or NotIn, the values array must be non-empty.
                              If the operator is Exists or DoesNotExist, the values

		{"----", "dir/obj", BucketNameInvalid{Bucket: "----"}},
		{"bucket", "", ObjectNameInvalid{Bucket: "bucket", Object: ""}},
		{"bucket", "doesnotexist", ObjectNotFound{Bucket: "bucket", Object: "doesnotexist"}},
		{"bucket", "dir/doesnotexist", ObjectNotFound{Bucket: "bucket", Object: "dir/doesnotexist"}},
		{"bucket", "dir", ObjectNotFound{Bucket: "bucket", Object: "dir"}},
		{"bucket", "dir/", ObjectNotFound{Bucket: "bucket", Object: "dir/"}},

                   (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 -->

  // operator represents a key's relationship to a set of values.
  // Valid operators are In, NotIn, Exists and DoesNotExist.
  optional string operator = 2;

  // values is an array of string values. If the operator is In or NotIn,
  // the values array must be non-empty. If the operator is Exists or DoesNotExist,
  // the values array must be empty. This array is replaced during a strategic
  // merge patch.
  // +optional

  // Represents a key's relationship to a set of values.
  // Valid operators are In, NotIn, Exists, DoesNotExist. Gt, and Lt.
  optional string operator = 2;

  // An array of string values. If the operator is In or NotIn,
  // the values array must be non-empty. If the operator is Exists or DoesNotExist,
  // the values array must be empty. If the operator is Gt or Lt, the values

        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();

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