Ensure that VM service extension handlers are run on outer event loop. (#17812)

Fixes https://github.com/flutter/flutter/issues/17597

packages/flutter/lib/src/foundation/binding.dart

@@ -363,6 +363,19 @@ abstract class BindingBase {
     final String methodName = 'ext.flutter.$name';
     developer.registerExtension(methodName, (String method, Map<String, String> parameters) async {
       assert(method == methodName);
+
+      // VM service extensions are handled as "out of band" messages by the VM,
+      // which means they are handled at various times, generally ASAP.
+      // Notably, this includes being handled in the middle of microtask loops.
+      // While this makes sense for some service extensions (e.g. "dump current
+      // stack trace", which explicitly doesn't want to wait for a loop to
+      // complete), Flutter extensions need not be handled with such high
+      // priority. Further, handling them with such high priority exposes us to
+      // the possibility that they're handled in the middle of a frame, which
+      // breaks many assertions. As such, we ensure they we run the callbacks
+      // on the outer event loop here.
+      await new Future<void>.delayed(Duration.zero);
+
       dynamic caughtException;
       StackTrace caughtStack;
       Map<String, dynamic> result;