Change Future.doWhile to use a loop for non-future results instead of recursion.

Avoids stack overflow on long sequences of non-future results.

R=floitsch@google.com

Review-Url: https://codereview.chromium.org/2853203002 .

sdk/lib/async/future.dart

@@ -485,8 +485,10 @@ abstract class Future<T> {
     var nextIteration;
     // Bind this callback explicitly so that each iteration isn't bound in the
     // context of all the previous iterations' callbacks.
+    // This avoids, e.g., deeply nested stack traces from the stack trace
+    // package.
     nextIteration = Zone.current.bindUnaryCallback((bool keepGoing) {
-      if (keepGoing) {
+      while (keepGoing) {
         FutureOr<bool> result;
         try {
           result = f();
@@ -500,10 +502,9 @@ abstract class Future<T> {
           result.then(nextIteration, onError: doneSignal._completeError);
           return;
         }
-        nextIteration(result);
-      } else {
-        doneSignal._complete(null);
+        keepGoing = result;
       }
+      doneSignal._complete(null);
     }, runGuarded: true);
     nextIteration(true);
     return doneSignal;

tests/lib/async/futures_test.dart

@@ -238,6 +238,30 @@ Future testDoWhileWithException() {
   });
 }
 
+Future testDoWhileManyFutures() {
+  int n = 100000;
+  var ftrue = new Future.value(false);
+  var ffalse = new Future.value(false);
+  return Future.doWhile(() {
+    return (--n > 0) ? ftrue : ffalse;
+  }).then((_) {
+    // Success
+  }, onError: (e, s) {
+    Expect.fail("$e\n$s");
+  });
+}
+
+Future testDoWhileManyValues() {
+  int n = 100000;
+  return Future.doWhile(() {
+    return (--n > 0);
+  }).then((_) {
+    // Success
+  }, onError: (e, s) {
+    Expect.fail("$e\n$s");
+  });
+}
+
 main() {
   List<Future> futures = new List<Future>();
 
@@ -259,10 +283,12 @@ main() {
   futures.add(testDoWhile());
   futures.add(testDoWhileSync());
   futures.add(testDoWhileWithException());
+  futures.add(testDoWhileManyFutures());
+  futures.add(testDoWhileManyValues());
 
   asyncStart();
   Future.wait(futures).then((List list) {
-    Expect.equals(18, list.length);
+    Expect.equals(20, list.length);
     asyncEnd();
   });
 }