JS backend: add simple test connected with frontend bug 'isStatement' for function body == 'hasBlockBody'

js/js.tests/test/org/jetbrains/k2js/test/semantics/FunctionTest.java

@@ -30,6 +30,10 @@ public class FunctionTest extends AbstractExpressionTest {
         fooBoxTest();
     }
 
+    public void testWhenFunction() throws Exception {
+        checkFooBoxIsOk();
+    }
+
     public void testFunctionLiteral() throws Exception {
         fooBoxTest();
     }

js/js.translator/testFiles/expression/function/cases/whenFunction.kt

@@ -0,0 +1,11 @@
+package foo
+
+fun foo(a: Int): Int = when {
+    a == 1 || a + 2 == 3 -> 5
+    else -> 6
+}
+fun box(): String {
+    if (foo(1) != 5) return "fail1: ${foo(1)}"
+    if (foo(2) != 6) return "fail2: ${foo(1)}"
+    return "OK"
+}