Fix error handling to include thrown voids.

src/dom.js

@@ -10,6 +10,7 @@ export type {Text, Key, TagName, DOM}
 */
 
 let driver/*:?Driver*/ = null
+const absent = new String("absent")
 
 class VirtualRoot /*::<model, action>*/ {
   /*::
@@ -29,7 +30,7 @@ class VirtualRoot /*::<model, action>*/ {
     this.address = address
   }
   renderWith(current/*:Driver*/) {
-    let exception = null
+    let exception = absent
     const previous = driver
     driver = current
 
@@ -42,7 +43,7 @@ class VirtualRoot /*::<model, action>*/ {
 
     driver = previous
 
-    if (exception != null) {
+    if (exception != absent) {
       throw exception
     }
   }

src/preemptive-animation-frame.js

@@ -18,6 +18,8 @@ let state/*:State*/ = NO_REQUEST
 let requests/*:Array<(time:Time) => any>*/ = []
 let ids/*:Array<number>*/ = []
 
+const absent = new String("absent")
+
 export const requestAnimationFrame = /*::<a>*/
   (request/*:(time:Time) => a*/) => {
     if (state === NO_REQUEST) {
@@ -76,7 +78,7 @@ const dispatchAnimationFrame = /*::<a>*/
   , index/*:number*/
   , time/*:Time*/
   ) => {
-    let exception = null
+    let exception = absent
     const count = requests.length
     try {
       while (index < count) {
@@ -93,7 +95,7 @@ const dispatchAnimationFrame = /*::<a>*/
       dispatchAnimationFrame(requests, index, time)
     }
 
-    if (exception != null) {
+    if (exception != absent) {
       throw exception
     }
   }

src/signal.js

@@ -9,6 +9,8 @@ export type {Signal, Address, Mailbox}
 export type {Translate, Reducer, AddressBook}
 */
 
+const absent = new String("absent")
+
 class Input /*::<a>*/ {
   /*::
   $type: "Signal.Signal";
@@ -29,7 +31,7 @@ class Input /*::<a>*/ {
     }
   }
   static notify(message/*:a*/, addressBook/*:AddressBook<a>*/, from/*:number*/, to/*:number*/)/*:void*/ {
-    let exception = null
+    let exception = absent
     try {
       while (from < to) {
         const address = addressBook[from]
@@ -47,7 +49,7 @@ class Input /*::<a>*/ {
       Input.notify(message, addressBook, from + 1, to)
     }
 
-    if (exception != null) {
+    if (exception != absent) {
       throw exception
     }
   }
@@ -82,7 +84,7 @@ class Input /*::<a>*/ {
         this.queue.push(value)
       }
     } else {
-      let exception = null
+      let exception = absent
       this.isBlocked = true
       try {
         this.value = value
@@ -101,7 +103,7 @@ class Input /*::<a>*/ {
         this.receive(value = this.queue.shift())
       }
 
-      if (exception != null) {
+      if (exception != absent) {
         throw exception
       }
     }