Make Zone API strong mode clean.

tests/lib_strong/async/async_await_zones_test.dart

Index: tests/lib_strong/async/async_await_zones_test.dart
diff --git a/tests/lib_strong/async/async_await_zones_test.dart b/tests/lib_strong/async/async_await_zones_test.dart
index cb7011caf896156b5b440a14b9644682a335bab0..fb3dc50c062e897abd081adebe0c00bdb797d193 100644
--- a/tests/lib_strong/async/async_await_zones_test.dart
+++ b/tests/lib_strong/async/async_await_zones_test.dart
@@ -94,7 +94,8 @@ increaseDepth() {
   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, () {
@@ -103,7 +104,8 @@ registerCallback(Zone self, ZoneDelegate parent, Zone zone, 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) {
@@ -112,7 +114,8 @@ registerUnaryCallback(Zone self, ZoneDelegate parent, Zone zone, f) {
   });
 }
 
-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) {
@@ -121,7 +124,7 @@ registerBinaryCallback(Zone self, ZoneDelegate parent, Zone zone, f) {
   });
 }
 
-sm(Zone self, ZoneDelegate parent, Zone zone, f) {
+void sm(Zone self, ZoneDelegate parent, Zone zone, f) {
   var oldDepth = depth;
   increaseDepth();
   return parent.scheduleMicrotask(zone, () {