dataflow to/from containers support

java/java-tests/testData/codeInsight/slice/backward/MapKeysTrack.java

@@ -0,0 +1,10 @@
+import java.util.*;
+
+class Map5 {
+    void f(String s, Map<String, String> m) {
+        m.put("x", s);
+        Set<String> keys = m.keySet();
+        String[] keyArray = keys.toArray(new String[0]);
+        String <caret>v = keyArray[0];
+    }
+}