JS backend: fix StringBuilder append #KT-4064 Fixed

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

@@ -62,4 +62,8 @@ public final class StandardClassesTest extends SingleFileTranslationTest {
     public void testArrayFactoryMethods() throws Exception {
         checkFooBoxIsOk();
     }
+
+    public void testStringBuilder() throws Exception {
+        checkFooBoxIsOk();
+    }
 }

js/js.translator/testFiles/kotlin_lib.js

@@ -510,6 +510,7 @@ String.prototype.contains = function (s) {
         }, {
         append:function (obj) {
             this.string = this.string + obj.toString();
+            return this;
         },
         toString:function () {
             return this.string;

js/js.translator/testFiles/standardClasses/cases/stringBuilder.kt

@@ -0,0 +1,13 @@
+package foo
+
+import java.util.StringBuilder
+
+fun box(): String {
+    val s = StringBuilder()
+    s.append("a")
+    s.append("b")?.append("c")
+    s.append("d")!!.append("e")
+
+    if (s.toString() != "abcde") return s.toString()
+    return "OK"
+}