Add edit-time support to AsyncCoroutineRunner

If the package com.unity.editorcoroutine is installed,
AsyncCoroutineRunner gains the ability to work at edit time.

In Unity 2019.1 and above, no additional setup is needed.
Otherwise, the symbol HAVE_EDITOR_COROUTINES must also be
manually defined.

Includes unit test.
Addresses part of modesttree#9

UnityProject/Assets/Plugins/AsyncAwaitUtil/Source/AsyncAwaitUtil.asmdef

@@ -0,0 +1,20 @@
+{
+    "name": "AsyncAwaitUtil",
+    "references": [
+        "GUID:478a2357cc57436488a56e564b08d223"
+    ],
+    "includePlatforms": [],
+    "excludePlatforms": [],
+    "allowUnsafeCode": false,
+    "overrideReferences": false,
+    "precompiledReferences": [],
+    "autoReferenced": true,
+    "defineConstraints": [],
+    "versionDefines": [
+        {
+            "name": "com.unity.editorcoroutines",
+            "expression": "0.0.2-preview",
+            "define": "HAVE_EDITOR_COROUTINES"
+        }
+    ]
+}

UnityProject/Assets/Plugins/AsyncAwaitUtil/Source/Internal/AsyncCoroutineRunner.cs

@@ -1,19 +1,49 @@
 using System;
 using System.Collections;
-using System.Collections.Generic;
-using System.Linq;
+
 using UnityEngine;
+#if UNITY_EDITOR && HAVE_EDITOR_COROUTINES
+using Unity.EditorCoroutines.Editor;
+#endif
 
 namespace UnityAsyncAwaitUtil
 {
-    public class AsyncCoroutineRunner : MonoBehaviour
+    public class AsyncCoroutineRunner : MonoBehaviour, AsyncCoroutineRunner.ICoroutineRunner
     {
-        static AsyncCoroutineRunner _instance;
+        public interface ICoroutineRunner
+        {
+            object StartCoroutine(IEnumerator routine);
+        }
+
+#if UNITY_EDITOR
+        class EditorAsyncCoroutineRunner : ICoroutineRunner
+        {
+            object ICoroutineRunner.StartCoroutine(IEnumerator routine)
+            {
+#if HAVE_EDITOR_COROUTINES
+                return EditorCoroutineUtility.StartCoroutine(routine, this);
+#elif UNITY_2019_1_OR_NEWER
+                throw new NotImplementedException("Install package com.unity.editorcoroutines");
+#else
+                // asmdef "Version Defines" support doesn't exist yet
+                throw new NotImplementedException("Install package com.unity.editorcoroutines and define HAVE_EDITOR_COROUTINES");
+#endif
+            }
+        }
+#endif
+
+        static ICoroutineRunner _instance;
 
-        public static AsyncCoroutineRunner Instance
+        public static ICoroutineRunner Instance
         {
             get
             {
+#if UNITY_EDITOR
+                if (_instance == null && !Application.isPlaying)
+                {
+                    _instance = new EditorAsyncCoroutineRunner();
+                }
+#endif
                 if (_instance == null)
                 {
                     _instance = new GameObject("AsyncCoroutineRunner")
@@ -31,5 +61,10 @@ void Awake()
 
             DontDestroyOnLoad(gameObject);
         }
+
+        object ICoroutineRunner.StartCoroutine(IEnumerator routine)
+        {
+            return StartCoroutine(routine);
+        }
     }
-}
+}

UnityProject/Assets/Plugins/AsyncAwaitUtil/Tests/Editor/TestMisc.cs

@@ -0,0 +1,31 @@
+using System.Collections;
+using System.Collections.Generic;
+
+using NUnit.Framework;
+using UnityEngine;
+using UnityEngine.TestTools;
+
+namespace UnityAsyncAwaitUtil
+{
+    class TestMisc
+    {
+        [UnityTest]
+        public IEnumerator TestAsyncCoroutineRunnerInEditor()
+        {
+            List<bool> result = new List<bool>();
+            AsyncCoroutineRunner.Instance.StartCoroutine(AppendCoroutine(result));
+            for (int i = 0; i < 10; ++i)
+            {
+                if (result.Count > 0) { yield break; }
+                yield return null;
+            }
+            Assert.Fail("Coroutine did not complete");
+        }
+
+        static IEnumerator AppendCoroutine(List<bool> result)
+        {
+            yield return null;
+            result.Add(true);
+        }
+    }
+}