Cleanup warnings, make them errors.

examples/falling_balls/build.gradle.kts

@@ -24,6 +24,8 @@ dependencies {
 
 tasks.withType<KotlinCompile>() {
     kotlinOptions.jvmTarget = "11"
+    kotlinOptions.allWarningsAsErrors = true
+    kotlinOptions.freeCompilerArgs += "-Xopt-in=kotlin.RequiresOptIn"
 }
 
 compose.desktop {

examples/falling_balls/src/main/kotlin/main.kt

@@ -1,8 +1,6 @@
 package org.jetbrains.compose.demo.falling
 
-import androidx.compose.desktop.Window
 import androidx.compose.ui.ExperimentalComposeUiApi
-import androidx.compose.ui.unit.IntSize
 import androidx.compose.ui.unit.dp
 import androidx.compose.ui.window.WindowSize
 import androidx.compose.ui.window.WindowState
@@ -10,7 +8,8 @@ import androidx.compose.ui.window.singleWindowApplication
 
 @OptIn(ExperimentalComposeUiApi::class)
 fun main() = singleWindowApplication(
-    title = "Falling Balls", state = WindowState(size = WindowSize(800.dp, 800.dp))) {
+    title = "Falling Balls", state = WindowState(size = WindowSize(800.dp, 800.dp))
-        FallingBallsGame()
+) {
-    }
+    FallingBallsGame()
+}