[FIR JS] Implement FirJsExternalChecker

The JsAllowValueClassesInExternals feature is enabled explicitly,
because otherwise it's enabled
implicitly depending on the backend. See:
org/jetbrains/kotlin/test/builders/LanguageVersionSettingsBuilder.kt:90

A property may have a fake source return kind, while its accessor
has a real source kind. In this case we can't "just copy"
the property return type down to the accessor.

compiler/fir/tree/src/org/jetbrains/kotlin/fir/Utils.kt

@@ -6,12 +6,9 @@
 package org.jetbrains.kotlin.fir
 
 import com.intellij.psi.PsiElement
-import org.jetbrains.kotlin.KtFakeSourceElementKind
-import org.jetbrains.kotlin.KtPsiSourceElement
-import org.jetbrains.kotlin.KtRealPsiSourceElement
+import org.jetbrains.kotlin.*
 import org.jetbrains.kotlin.descriptors.Modality
 import org.jetbrains.kotlin.descriptors.Visibility
-import org.jetbrains.kotlin.fakeElement
 import org.jetbrains.kotlin.fir.declarations.FirContextReceiver
 import org.jetbrains.kotlin.fir.declarations.FirDeclarationStatus
 import org.jetbrains.kotlin.fir.declarations.FirFile
@@ -32,11 +29,15 @@ import org.jetbrains.kotlin.util.wrapIntoSourceCodeAnalysisExceptionIfNeeded
 // TODO: rewrite
 fun FirBlock.returnExpressions(): List<FirExpression> = listOfNotNull(statements.lastOrNull() as? FirExpression)
 
-// do we need a deep copy here ?
 fun <R : FirTypeRef> R.copyWithNewSourceKind(newKind: KtFakeSourceElementKind): R {
     if (source == null) return this
     if (source?.kind == newKind) return this
-    val newSource = source?.fakeElement(newKind)
+    return copyWithNewSource(source?.fakeElement(newKind))
+}
+
+// do we need a deep copy here ?
+fun <R : FirTypeRef> R.copyWithNewSource(newSource: KtSourceElement?): R {
+    if (source?.kind == newSource?.kind) return this
 
     @Suppress("UNCHECKED_CAST")
     return when (val typeRef = this) {
@@ -63,7 +64,7 @@ fun <R : FirTypeRef> R.copyWithNewSourceKind(newKind: KtFakeSourceElementKind):
             isMarkedNullable = typeRef.isMarkedNullable
             annotations += typeRef.annotations
         }
-        is FirImplicitBuiltinTypeRef -> typeRef.withFakeSource(newKind)
+        is FirImplicitBuiltinTypeRef -> typeRef.withNewSource(newSource)
         is FirIntersectionTypeRef -> buildIntersectionTypeRef {
             source = newSource
             isMarkedNullable = typeRef.isMarkedNullable

compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/utils/FirDeclarationUtil.kt

@@ -43,3 +43,5 @@ val FirDeclaration.isNonLocal
         is FirClassLikeDeclaration -> !symbol.classId.isLocal
         else -> false
     }
+
+val FirCallableDeclaration.isExtension get() = receiverParameter != null

compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/impl/FirImplicitBuiltinTypeRef.kt

@@ -183,10 +183,9 @@ class FirImplicitKMutableProperty2TypeRef(
     arrayOf(dispatchReceiverTypeArgument, extensionReceiverTypeArgument, propertyTypeArgument)
 )
 
-fun FirImplicitBuiltinTypeRef.withFakeSource(kind: KtFakeSourceElementKind): FirImplicitBuiltinTypeRef {
+fun FirImplicitBuiltinTypeRef.withNewSource(newSource: KtSourceElement?): FirImplicitBuiltinTypeRef {
     val source = source ?: return this
-    if (source.kind == kind) return this
-    val newSource = source.fakeElement(kind)
+    if (source.kind == newSource?.kind) return this
     return when (this) {
         is FirImplicitUnitTypeRef -> FirImplicitUnitTypeRef(newSource)
         is FirImplicitAnyTypeRef -> FirImplicitAnyTypeRef(newSource)