Fix type of FakeMessageChannel

so that the type system thinks it's a drop in replacement for MessageChannel

Author: hoodmane

src/fake_message_channel.ts

@@ -32,7 +32,7 @@ class FakeMessagePort {
   }
 }
 
-export class FakeMessageChannel {
+class FakeMessageChannel {
   port1: FakeMessagePort;
   port2: FakeMessagePort;
   constructor() {
@@ -41,4 +41,8 @@ export class FakeMessageChannel {
     this.port1._otherPort = this.port2;
     this.port2._otherPort = this.port1;
   }
-}
+};
+
+let FakeMessageChannel1 = FakeMessageChannel as unknown as typeof MessageChannel;
+
+export { FakeMessageChannel1 as FakeMessageChannel};