Emulate java.util.concurrent.Flow

Bug: #9547
Bug-Link: https://github.com/gwtproject/gwt/issues/9547
Change-Id: I0e01ee3737ebee118db2b2e023cbe02beb9f2347

user/super/com/google/gwt/emul/java/util/concurrent/Flow.java

diff --git a/user/super/com/google/gwt/emul/java/util/concurrent/Flow.java b/user/super/com/google/gwt/emul/java/util/concurrent/Flow.java
new file mode 100644
index 0000000..a23b8b7
--- /dev/null
+++ b/user/super/com/google/gwt/emul/java/util/concurrent/Flow.java
@@ -0,0 +1,73 @@
+/*
+ * Copyright 2018 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package java.util.concurrent;
+
+/**
+ * Emulation of Flow.
+ */
+public final class Flow {
+
+  private Flow() { }
+
+  /**
+   * Emulation of Publisher.
+   *
+   * @param <T> the published item type
+   */
+  @FunctionalInterface
+  public interface Publisher<T> {
+    void subscribe(Subscriber<? super T> subscriber);
+  }
+
+  /**
+   * Emulation of Subscriber.
+   *
+   * @param <T> the subscribed item type
+   */
+  public interface Subscriber<T> {
+    void onSubscribe(Subscription subscription);
+
+    void onNext(T item);
+
+    void onError(Throwable throwable);
+
+    void onComplete();
+  }
+
+  /**
+   * Emulation of Subscription.
+   */
+  public interface Subscription {
+    void request(long n);
+
+    void cancel();
+  }
+
+  /**
+   * Emulation of Processor.
+   *
+   * @param <T> the subscribed item type
+   * @param <R> the published item type
+   */
+  public interface Processor<T,R> extends Subscriber<T>, Publisher<R> {
+  }
+
+  private static final int DEFAULT_BUFFER_SIZE = 256;
+
+  public static int defaultBufferSize() {
+    return DEFAULT_BUFFER_SIZE;
+  }
+}