Sprinkle some types on the SDK like the static typing fairy.

pkg/dev_compiler/tool/input_sdk/lib/async/stream_impl.dart

Index: pkg/dev_compiler/tool/input_sdk/lib/async/stream_impl.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/lib/async/stream_impl.dart b/pkg/dev_compiler/tool/input_sdk/lib/async/stream_impl.dart
index dffaa1205910b222ea7ca6bbdbf197069f33cccd..7099973bebfb77a974bb4c53f6bb6ae8b00b2e05 100644
--- a/pkg/dev_compiler/tool/input_sdk/lib/async/stream_impl.dart
+++ b/pkg/dev_compiler/tool/input_sdk/lib/async/stream_impl.dart
@@ -401,7 +401,7 @@ class _BufferingStreamSubscription<T> implements StreamSubscription<T>,
    * during the call, and it checks for state changes after the call
    * that should cause further callbacks.
    */
-  void _guardCallback(callback) {
+  void _guardCallback(void callback()) {
     assert(!_inCallback);
     bool wasInputPaused = _isInputPaused;
     _state |= _STATE_IN_CALLBACK;