JS backend: added String.isEmpty

(cherry picked from commit 6cf1761)

js/js.libraries/src/core/string.kt

@@ -40,6 +40,8 @@ public val String.size: Int
 
 public fun String.length(): Int = js.noImpl
 
+public fun String.isEmpty(): Boolean = js.noImpl
+
 /*
 
 native public fun String.equalsIgnoreCase(anotherString: String) : Boolean = (this as java.lang.String).equalsIgnoreCase(anotherString)
@@ -118,7 +120,7 @@ native public fun String.getBytes(charset : java.nio.charset.Charset) : ByteArra
 
 native public fun String.getBytes(charsetName : String) : ByteArray = (this as java.lang.String).getBytes(charsetName)!!
 
-native public fun String.getChars(srcBegin : Int, srcEnd : Int, dst : CharArray, dstBegin : Int) : Unit = (this as java.lang.String).getChars(srcBegin, srcEnd, dst, dstBegin)!!
+native public fun String.getChars(srcBegin : Int, srcEnd : Int, dst : CharArray, dstBegin : Int) : Tuple0 = (this as java.lang.String).getChars(srcBegin, srcEnd, dst, dstBegin)!!
 
 native public fun String.intern() : String = (this as java.lang.String).intern()!!
 

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

@@ -16,9 +16,17 @@
 
 package org.jetbrains.k2js.translate.intrinsic.functions.factories;
 
+import com.google.dart.compiler.backend.js.ast.JsExpression;
+import com.google.dart.compiler.backend.js.ast.JsNameRef;
 import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+import org.jetbrains.k2js.translate.context.TranslationContext;
 import org.jetbrains.k2js.translate.intrinsic.functions.basic.BuiltInFunctionIntrinsic;
 import org.jetbrains.k2js.translate.intrinsic.functions.basic.BuiltInPropertyIntrinsic;
+import org.jetbrains.k2js.translate.intrinsic.functions.basic.FunctionIntrinsic;
+import org.jetbrains.k2js.translate.utils.JsAstUtils;
+
+import java.util.List;
 
 import static org.jetbrains.k2js.translate.intrinsic.functions.patterns.PatternBuilder.pattern;
 
@@ -34,5 +42,15 @@ public final class StringOperationFIF extends CompositeFIF {
         add(pattern("js", "<get-size>").receiverExists(true), lengthIntrinsic);
         add(pattern("js", "length").receiverExists(true), lengthIntrinsic);
         add(pattern("jet", "CharSequence", "<get-length>"), lengthIntrinsic);
+        add(pattern("js", "isEmpty").receiverExists(true), new FunctionIntrinsic() {
+            @NotNull
+            @Override
+            public JsExpression apply(
+                    @Nullable JsExpression receiver, @NotNull List<JsExpression> arguments, @NotNull TranslationContext context
+            ) {
+                assert receiver != null;
+                return JsAstUtils.equality(new JsNameRef("length", receiver), context.program().getNumberLiteral(0));
+            }
+        });
     }
 }

js/js.translator/testFiles/expression/string/cases/extensionMethods.kt

@@ -2,5 +2,5 @@ package foo
 
 fun box(): Boolean {
     val s = "bar"
-    return s.size == 3 && s.length() == 3 && s.length == 3
-}
+    return s.size == 3 && s.length() == 3 && s.length == 3 && s.startsWith("b") && s.endsWith("r") && s.contains("a") && !s.isEmpty()
+}