Add CharSequence.subSequence

This is done primarily for JVM interoperability, otherwise it's impossible to
inherit from CharSequence there. On JS subSequence at the moment just invokes
substring.

 #KT-5956 Fixed

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

@@ -90,4 +90,8 @@ public final class StringTest extends AbstractExpressionTest {
     public void testNullableTypeInStringTemplate() throws Exception {
         checkFooBoxIsOk();
     }
+
+    public void testSubSequence() throws Exception {
+        checkFooBoxIsOk();
+    }
 }

js/js.translator/src/org/jetbrains/k2js/translate/intrinsic/functions/factories/StringOperationFIF.java

@@ -30,6 +30,7 @@ public final class StringOperationFIF extends CompositeFIF {
         // This intrinsic is needed because charAt is a public function taking one parameter and thus its name would be mangled otherwise
         add(pattern("kotlin", "CharSequence", "charAt").checkOverridden(), new BuiltInFunctionIntrinsic("charAt"));
         add(pattern("kotlin", "CharSequence", "length").checkOverridden(), LENGTH_PROPERTY_INTRINSIC);
+        add(pattern("kotlin", "CharSequence", "subSequence").checkOverridden(), new BuiltInFunctionIntrinsic("substring"));
         add(pattern("kotlin.js", "isEmpty").isExtensionOf("kotlin.CharSequence"), IS_EMPTY_INTRINSIC);
     }
 }

js/js.translator/testData/expression/string/cases/subSequence.kt

@@ -0,0 +1,15 @@
+package foo
+
+fun box(): String {
+    val kotlin: String = "kotlin"
+
+    if (kotlin.subSequence(0, kotlin.length()) != kotlin) return "Fail 0"
+
+    val kot: CharSequence = kotlin.subSequence(0, 3)
+    if (kot.toString() != "kot") return "Fail 1: $kot"
+
+    val tlin = (kotlin : CharSequence).subSequence(2, 6)
+    if (tlin.toString() != "tlin") return "Fail 2: $tlin"
+
+    return "OK"
+}