[Wasm] Support defineExternally

compiler/ir/backend.wasm/src/org/jetbrains/kotlin/backend/wasm/lower/FieldInitializersLowering.kt

@@ -39,6 +39,10 @@ class FieldInitializersLowering(val context: WasmBackendContext) : FileLoweringP
 
             override fun visitField(declaration: IrField) {
                 super.visitField(declaration)
+
+                // External properties can be "initialized" with `= defineExternally`. Ignoring it.
+                if (declaration.isExternal) return
+
                 if (!declaration.isStatic) return
                 val initValue: IrExpression = declaration.initializer?.expression ?: return
 

libraries/stdlib/wasm/src/kotlin/js/definedExternally.kt

@@ -0,0 +1,37 @@
+/*
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package kotlin.js
+
+import kotlin.wasm.internal.ExcludedFromCodegen
+
+/**
+ * The property that can be used as a placeholder for statements and values that are defined in JavaScript.
+ *
+ * This property can be used in two cases:
+ *
+ *   * To represent body of an external function. In most cases Kotlin does not require to provide bodies of external
+ *     functions and properties, but if for some reason you want to (for example, due to limitation of your coding style guides),
+ *     you should use `definedExternally`.
+ *   * To represent value of default argument.
+ *
+ * There's two forms of using `definedExternally`:
+ *
+ *   1. `= definedExternally` (for functions, properties and parameters).
+ *   2. `{ definedExternally }` (for functions and property getters/setters).
+ *
+ * This property can't be used from normal code.
+ *
+ * Examples:
+ *
+ * ``` kotlin
+ * external fun foo(): String = definedExternally
+ * external fun bar(x: Int) { definedExternally }
+ * external fun baz(z: Any = definedExternally): Array<Any>
+ * external val prop: Float = definedExternally
+ * ```
+ */
+@ExcludedFromCodegen
+public external val definedExternally: Nothing