Make generic type of timeout-future the same as the one from the original.

R=lrn@google.com

Review URL: https://codereview.chromium.org//1299443002 .

sdk/lib/async/future.dart

@@ -546,11 +546,12 @@ abstract class Future<T> {
    * If this future does not complete before `timeLimit` has passed,
    * the [onTimeout] action is executed instead, and its result (whether it
    * returns or throws) is used as the result of the returned future.
+   * The [onTimeout] function must return a [T] or a `Future<T>`.
    *
    * If `onTimeout` is omitted, a timeout will cause the returned future to
    * complete with a [TimeoutException].
    */
-  Future timeout(Duration timeLimit, {onTimeout()});
+  Future<T> timeout(Duration timeLimit, {onTimeout()});
 }
 
 /**

sdk/lib/async/future_impl.dart

@@ -680,9 +680,9 @@ class _Future<T> implements Future<T> {
     }
   }
 
-  Future timeout(Duration timeLimit, {onTimeout()}) {
+  Future<T> timeout(Duration timeLimit, {onTimeout()}) {
     if (_isComplete) return new _Future.immediate(this);
-    _Future result = new _Future();
+    _Future result = new _Future<T>();
     Timer timer;
     if (onTimeout == null) {
       timer = new Timer(timeLimit, () {

tests/lib/async/future_timeout_test.dart

@@ -185,4 +185,14 @@ main() {
       expect(s, null);
     }));
   });
+
+  test("timeoutType", () {
+    Completer completer = new Completer<int>();
+    Future timedOut = completer.future.timeout(
+        const Duration(milliseconds: 5));
+    expect(timedOut, new isInstanceOf<Future<int>>());
+    expect(timedOut, isNot(new isInstanceOf<Future<String>>()));
+    timedOut.catchError((_) {});
+    completer.complete(499);
+  });
 }