[JS IR] add a flag which enable safe property access

If an accessor is not available at runtime we fall back
to the property access.

This is useful in cases when JS objects are casted to Kotlin
classes implicitly. This pattern did work in the old BE, which
lead to a significant amount of code which doesn't work anymore.

compiler/cli/cli-common/src/org/jetbrains/kotlin/cli/common/arguments/K2JSCompilerArguments.kt

@@ -151,6 +151,9 @@ class K2JSCompilerArguments : CommonCompilerArguments() {
     )
     var irModuleName: String? by NullableStringFreezableVar(null)
 
+    @Argument(value = "-Xir-legacy-property-access", description = "Force property access via JS properties (requires -Xir-export-all)")
+    var irLegacyPropertyAccess: Boolean by FreezableVar(false)
+
     @Argument(value = "-Xir-per-module", description = "Splits generated .js per-module")
     var irPerModule: Boolean by FreezableVar(false)