Emulating CompletableFuture with Promise

I'm trying to emulate CompletableFuture by backing directly onto Promise, but when compiling it gives this error: 

"No source code is available for type java.util.concurrent.CompletableFuture<T>"

(it finds other JRE classes I've replaced fine)

My source I'm including is below. Am I doing anything illegal?

********************************************

package java.util.concurrent;

import com.google.gwt.user.client.*;

import jsinterop.annotations.*;

import java.util.function.*;

@JsType(isNative=true, name = "Promise", namespace = JsPackage.GLOBAL)

public class CompletableFuture<T> implements CompletionStage<T> {

    @JsMethod(name = "then")

    public native <U> CompletableFuture<U> thenApply(Function<? super T,? extends U> fn);

    @JsMethod(name = "then")

    public native CompletableFuture<Void> thenAccept(Consumer<? super T> action);

    @JsMethod(name = "then")

    public native <U> CompletableFuture<U> thenCompose(Function<? super T, ? extends CompletionStage<U>> fn);

    @JsMethod(name = "reject")

    public native boolean completeExceptionally(Throwable ex);

    @JsMethod(name = "resolve")

    public native boolean complete(T value);

    @JsMethod(name = "resolve")

    public static native <U> CompletableFuture<U> completedFuture(U value);

    @JsOverlay

    public T get() throws InterruptedException, ExecutionException {

        Window.alert("Calling synchronous get() on CompletableFuture is not possible in Javascript!");

        throw new IllegalStateException("Unimplemented!");

    }

}

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