refs spotbugs#60: reproduce reported problem

Author: KengoTODA

findbugs/src/test/java/edu/umd/cs/findbugs/detect/FindUnsatisfiedObligationTest.java

@@ -0,0 +1,22 @@
+package edu.umd.cs.findbugs.detect;
+
+import static org.junit.Assert.assertThat;
+
+import static org.hamcrest.core.Is.is;
+
+import static org.hamcrest.collection.IsEmptyIterable.*;
+import org.junit.Test;
+
+import edu.umd.cs.findbugs.AbstractIntegrationTest;
+
+public class FindUnsatisfiedObligationTest extends AbstractIntegrationTest {
+    /**
+     * @see <a href="https://github.com/spotbugs/spotbugs/issues/60">GitHub
+     *      issue</a>
+     */
+    @Test
+    public void testIssue60() {
+        performAnalysis("Issue60.class");
+        assertThat(getBugCollection(), is(emptyIterable()));
+    }
+}

findbugsTestCases/src/java/Issue60.java

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+import java.io.IOException;
+import java.io.InputStream;
+import java.net.URL;
+import java.util.Properties;
+import java.util.stream.Stream;
+
+public class Issue60 {
+
+    public static void create(URL url) {
+        try (InputStream in = url.openStream()) {
+            Properties p1 = new Properties();
+            p1.load(in);
+        } catch (IOException e) {
+        }
+    }
+
+    public Stream<String> keys() {
+        return Stream.<Properties> of()
+                .flatMap(p -> p.stringPropertyNames().stream());
+    }
+
+}