KJS: improve declaration for JS Promise and add helpers to simplify some usecases

* add `then` with only one parameter to make usages from Kotlin more idiomatic
* add overload static `resolve` which accepts Promise<S> according to the spec
* add helper functions for `then` to simplify chained usages
  when Promise is returned from `then` or `catch` (workaround for KT-19672)

js/js.libraries/src/core/promise.kt

@@ -16,10 +16,19 @@
 
 package kotlin.js
 
+import kotlin.internal.LowPriorityInOverloadResolution
+
 /**
  * Exposes the JavaScript [Promise object](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Promise) to Kotlin.
  */
 public open external class Promise<out T>(executor: (resolve: (T) -> Unit, reject: (Throwable) -> Unit) -> Unit) {
+    @LowPriorityInOverloadResolution
+    public open fun <S> then(onFulfilled: ((T) -> S)?): Promise<S>
+    @LowPriorityInOverloadResolution
+    public open fun <S> then(onFulfilled: ((T) -> S)?, onRejected: ((Throwable) -> S)?): Promise<S>
+
+    public open fun <S> catch(onRejected: (Throwable) -> S): Promise<S>
+
     companion object {
         public fun <S> all(promise: Array<out Promise<S>>): Promise<Array<out S>>
 
@@ -28,9 +37,20 @@ public open external class Promise<out T>(executor: (resolve: (T) -> Unit, rejec
         public fun reject(e: Throwable): Promise<Nothing>
 
         public fun <S> resolve(e: S): Promise<S>
+        public fun <S> resolve(e: Promise<S>): Promise<S>
     }
+}
 
-    public open fun <S> then(onFulfilled: ((T) -> S)?, onRejected: ((Throwable) -> S)? = definedExternally): Promise<S>
+// It's workaround for KT-19672 since we can fix it properly until KT-11265 isn't fixed.
+inline fun <T, S> Promise<Promise<T>>.then(
+        noinline onFulfilled: ((T) -> S)?
+): Promise<S> {
+    return this.unsafeCast<Promise<T>>().then(onFulfilled)
+}
 
-    public open fun <S> catch(onRejected: (Throwable) -> S): Promise<S>
-}
+inline fun <T, S> Promise<Promise<T>>.then(
+        noinline onFulfilled: ((T) -> S)?,
+        noinline onRejected: ((Throwable) -> S)?
+): Promise<S> {
+    return this.unsafeCast<Promise<T>>().then(onFulfilled, onRejected)
+}