Fix name clash in JS implementation of multiplatform extension property

See KT-18756

compiler/testData/multiplatform/jsNameClash/common.kt

@@ -0,0 +1,2 @@
+header class ClassWithImplByExtension
+header val ClassWithImplByExtension.extensionVal: Int

compiler/testData/multiplatform/jsNameClash/js.kt

@@ -0,0 +1,3 @@
+impl class ClassWithImplByExtension
+impl val ClassWithImplByExtension.extensionVal: Int
+    get() = 0

compiler/testData/multiplatform/jsNameClash/output.txt

@@ -0,0 +1,7 @@
+-- Common --
+Exit code: OK
+Output:
+
+-- JS --
+Exit code: OK
+Output:

compiler/tests/org/jetbrains/kotlin/multiplatform/MultiPlatformIntegrationTestGenerated.java

@@ -96,6 +96,12 @@ public class MultiPlatformIntegrationTestGenerated extends AbstractMultiPlatform
         doTest(fileName);
     }
 
+    @TestMetadata("jsNameClash")
+    public void testJsNameClash() throws Exception {
+        String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/multiplatform/jsNameClash/");
+        doTest(fileName);
+    }
+
     @TestMetadata("missingOverload")
     public void testMissingOverload() throws Exception {
         String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/multiplatform/missingOverload/");

js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/JsNameClashChecker.kt

@@ -110,10 +110,10 @@ class JsNameClashChecker : SimpleDeclarationChecker {
     }
 
     private val DeclarationDescriptor.isActual: Boolean
-        get() = this is MemberDescriptor && this.isActual
+        get() = this is MemberDescriptor && this.isActual || this is PropertyAccessorDescriptor && this.correspondingProperty.isActual
 
     private val DeclarationDescriptor.isExpect: Boolean
-        get() = this is MemberDescriptor && this.isExpect
+        get() = this is MemberDescriptor && this.isExpect || this is PropertyAccessorDescriptor && this.correspondingProperty.isExpect
 
     private fun isFakeOverridingNative(descriptor: CallableMemberDescriptor): Boolean {
         return descriptor.kind == CallableMemberDescriptor.Kind.FAKE_OVERRIDE &&