Make Zone API strong mode clean.

tests/lib/async/async_await_zones_test.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // Test that async functions don't zone-register their callbacks for each
 // await. Async functions should register their callback once in the beginning
 // and then reuse it for all awaits in their body.
 // This has two advantages: it is faster, when there are several awaits (on
 // the Future class from dart:async), and it avoids zone-nesting when tracing
 // stacktraces.
@@ skipping 76 matching lines @@
 increaseDepth() {
   depthIncreases++;
   depth++;
   // The async/await code should not register callbacks recursively in the
   // then-calls. As such the depth should never grow too much. We don't want
   // to commit to a specific value, since implementations still have some
   // room in how async/await is implemented, but 20 should be safe.
   Expect.isTrue(depth < 20);
 }
 
-registerCallback(Zone self, ZoneDelegate parent, Zone zone, f) {
+ZoneCallback<R> registerCallback<R>(
+    Zone self, ZoneDelegate parent, Zone zone, R f()) {
   var oldDepth = depth;
   increaseDepth();
   return parent.registerCallback(zone, () {
     depth = oldDepth;
     return f();
   });
 }
 
-registerUnaryCallback(Zone self, ZoneDelegate parent, Zone zone, f) {
+ZoneUnaryCallback<R, T> registerUnaryCallback<R, T>(
+    Zone self, ZoneDelegate parent, Zone zone, R f(T arg)) {
   var oldDepth = depth;
   increaseDepth();
   return parent.registerUnaryCallback(zone, (x) {
     depth = oldDepth;
     return f(x);
   });
 }
 
-registerBinaryCallback(Zone self, ZoneDelegate parent, Zone zone, f) {
+ZoneBinaryCallback<R, T1, T2> registerBinaryCallback<R, T1, T2>(
+    Zone self, ZoneDelegate parent, Zone zone, R f(T1 arg1, T2 arg2)) {
   var oldDepth = depth;
   increaseDepth();
   return parent.registerBinaryCallback(zone, (x, y) {
     depth = oldDepth;
     return f(x, y);
   });
 }
 
 sm(Zone self, ZoneDelegate parent, Zone zone, f) {
   var oldDepth = depth;
   increaseDepth();
   return parent.scheduleMicrotask(zone, () {
     depth = oldDepth;
     return f();
   });
 }
 
 main() {
   asyncStart();
   var desc = new ZoneSpecification(
       registerCallback: registerCallback,
       registerUnaryCallback: registerUnaryCallback,
       registerBinaryCallback: registerBinaryCallback,
       scheduleMicrotask: sm);
   var future = runZoned(runTests, zoneSpecification: desc);
   future.then((_) => asyncEnd());
 }