Don't double-complete in host.dart.

lib/src/runner/browser/static/host.dart.js

Index: lib/src/runner/browser/static/host.dart.js
diff --git a/lib/src/runner/browser/static/host.dart.js b/lib/src/runner/browser/static/host.dart.js
index 641b0169f3187e95b4ffce19da73d5c6bc6aa9a3..d2d9ec9edee3a42a377647485ce7e33b7119c811 100644
--- a/lib/src/runner/browser/static/host.dart.js
+++ b/lib/src/runner/browser/static/host.dart.js
@@ -12772,7 +12772,7 @@ var dart = [
 ["test.multi_channel", "package:test/src/util/multi_channel.dart", , V, {
   "^": "",
   _MultiChannel: {
-    "^": "StreamChannelMixin;_innerStream,_innerSink,_innerStreamSubscription,_streamController,_sinkController,_streamControllers,_sinkControllers,_nextId",
+    "^": "StreamChannelMixin;_innerStream,_innerSink,_innerStreamSubscription,_closed,_streamController,_sinkController,_streamControllers,_sinkControllers,_nextId",
     virtualChannel$1: function(id) {
       var t1, t2, inputId, t3, streamController, sinkController;
       t1 = {};
@@ -12804,6 +12804,9 @@ var dart = [
     },
     _closeChannel$2: function(inputId, outputId) {
       var t1, t2;
+      if (this._closed)
+        return;
+      this._closed = true;
       t1 = this._streamControllers;
       J.close$0$x(t1.remove$1(0, inputId));
       J.close$0$x(this._sinkControllers.remove$1(0, inputId));
@@ -12835,7 +12838,7 @@ var dart = [
       this._innerStreamSubscription = this._innerStream.listen$3$onDone$onError(new V._MultiChannel_closure1(this), this.get$_closeInnerChannel(), t1.get$addError());
     },
     static: {_MultiChannel$: function(_innerStream, _innerSink) {
-        var t1 = new V._MultiChannel(_innerStream, _innerSink, null, P.StreamController_StreamController(null, null, null, null, true, null), P.StreamController_StreamController(null, null, null, null, true, null), P.LinkedHashMap_LinkedHashMap(null, null, null, P.$int, P.StreamController), P.LinkedHashMap_LinkedHashMap(null, null, null, P.$int, P.StreamController), 1);
+        var t1 = new V._MultiChannel(_innerStream, _innerSink, null, false, P.StreamController_StreamController(null, null, null, null, true, null), P.StreamController_StreamController(null, null, null, null, true, null), P.LinkedHashMap_LinkedHashMap(null, null, null, P.$int, P.StreamController), P.LinkedHashMap_LinkedHashMap(null, null, null, P.$int, P.StreamController), 1);
         t1._MultiChannel$2(_innerStream, _innerSink);
         return t1;
       }}
@@ -12991,7 +12994,9 @@ var dart = [
       if (t1._state >= 4)
         H.throwExpression(t1._badEventState$0());
       t1._async$_add$1(t2);
-      this._captured_readyCompleter_2.complete$0(0);
+      t1 = this._captured_readyCompleter_2;
+      if (t1.future._state === 0)
+        t1.complete$0(0);
     }
   },
   _connectToIframe_closure0: {