[Wasm] Use Unit in IDL event handler return type

Return value can be used in rare cases to stop event propagation.
Event methods can be called instead.

^KT-57058 Fixed

libraries/tools/dukat/src/main/kotlin/org/jetbrains/kotlin/tools/dukat/wasm/translator.kt

@@ -8,7 +8,6 @@ package org.jetbrains.kotlin.tools.dukat.wasm
 import org.jetbrains.dukat.astCommon.IdentifierEntity
 import org.jetbrains.dukat.astCommon.NameEntity
 import org.jetbrains.dukat.astModel.*
-import org.jetbrains.dukat.astModel.modifiers.VisibilityModifierModel
 import org.jetbrains.dukat.commonLowerings.AddExplicitGettersAndSetters
 import org.jetbrains.dukat.idlLowerings.*
 import org.jetbrains.dukat.idlParser.parseIDL
@@ -19,6 +18,7 @@ import org.jetbrains.dukat.ownerContext.NodeOwner
 fun translateIdlToSourceSet(fileName: String): SourceSetModel {
     val translationContext = TranslationContext()
     return parseIDL(fileName, DirectoryReferencesResolver())
+        .voidifyEventHandlerReturnType()
         .resolvePartials()
         .addConstructors()
         .resolveTypedefs()

libraries/tools/dukat/src/main/kotlin/org/jetbrains/kotlin/tools/dukat/wasm/voidifyEventHandlerReturnType.kt

@@ -0,0 +1,20 @@
+package org.jetbrains.kotlin.tools.dukat.wasm
+
+import org.jetbrains.dukat.idlDeclarations.*
+import org.jetbrains.dukat.idlLowerings.IDLLowering
+
+private class VoidifyEventHandlerReturnType : IDLLowering {
+    override fun lowerTypedefDeclaration(declaration: IDLTypedefDeclaration, owner: IDLFileDeclaration): IDLTypedefDeclaration {
+        if (declaration.name == "EventHandlerNonNull") {
+            val type = declaration.typeReference as IDLFunctionTypeDeclaration
+            return declaration.copy(typeReference = type.copy(returnType = voidType))
+        }
+        return super.lowerTypedefDeclaration(declaration, owner)
+    }
+}
+
+private val voidType = IDLSingleTypeDeclaration("void", null, false)
+
+fun IDLSourceSetDeclaration.voidifyEventHandlerReturnType(): IDLSourceSetDeclaration {
+    return VoidifyEventHandlerReturnType().lowerSourceSetDeclaration(this)
+}