SI-9583: Update SystemProperties.empty to return a mutable.Map to fix builders

Author: ShaneDelmore

src/library/scala/sys/SystemProperties.scala

@@ -32,7 +32,7 @@ class SystemProperties
 extends mutable.AbstractMap[String, String]
    with mutable.Map[String, String] {
 
-  override def empty = new SystemProperties
+  override def empty = mutable.Map[String, String]()
   override def default(key: String): String = null
 
   def iterator: Iterator[(String, String)] = wrapAccess {

test/junit/scala/util/SystemPropertiesTest.scala

@@ -0,0 +1,27 @@
+package scala.util
+
+import org.junit.runner.RunWith
+import org.junit.runners.JUnit4
+import org.junit.Test
+import org.junit.Assert._
+
+@RunWith(classOf[JUnit4])
+class SystemPropertiesTest {
+  @Test
+  def filterAll(): Unit = {
+    val isEmpty = sys.props.filter(_ => false).size == 0
+    assertTrue("A filter matching nothing should produce an empty result", isEmpty)
+  }
+
+  @Test
+  def filterNone(): Unit = {
+    val isUnchanged = sys.props.filter(_ => true) == sys.props
+    assertTrue("A filter matching everything should not change the result", isUnchanged)
+  }
+
+  @Test
+  def empty(): Unit = {
+    val hasSize0 = sys.props.empty.size == 0
+    assertTrue("SystemProperties.empty should have size of 0", hasSize0)
+  }
+}