Java slicing: support -> this/-> paramN contracts

Fixes IDEA-121571 Feature request: A Contract Annotation that indicates that a method returns its argument

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

@@ -0,0 +1,21 @@
+class Test {
+
+  private String <flown1>s;
+  private String s2;
+
+  void test(String <flown112111><flown1111>s, String s2) {
+    this.s = <flown11>requireNonNull(<flown11211><flown111>s);
+    this.s2 = requireNonNull(s2);
+  }
+
+  String foo() {
+    return <caret>s;
+  }
+
+  public static <T> T requireNonNull(T <flown1121>obj) {
+    if (obj == null)
+      throw new NullPointerException();
+    return <flown112>obj;
+  }
+
+}