Fix datagram issue where available is not correctly updated upon receive.

runtime/bin/socket_patch.dart

Index: runtime/bin/socket_patch.dart
diff --git a/runtime/bin/socket_patch.dart b/runtime/bin/socket_patch.dart
index 23e3f9e200269a9b5423cf64a983c5330fc1af31..d598e273825b799e08ea8fed332508482b1d43c3 100644
--- a/runtime/bin/socket_patch.dart
+++ b/runtime/bin/socket_patch.dart
@@ -568,13 +568,11 @@ class _NativeSocket extends _NativeSocketNativeWrapper with _ServiceObject {
       return null;
     }
     if (result != null) {
-      if (Platform.isMacOS) {
-        // Mac includes the header size, so we need to query the actual
-        // available.
-        available = nativeAvailable();
-      } else {
-        available -= result.data.length;
-      }
+      // Read the next available. Available is only for the next datagram, not
+      // the sum of all datagrams pending, so we need to call after each
+      // receive. If available becomes > 0, the _NativeSocket will continue to
+      // emit read events.
+      available = nativeAvailable();
       totalRead += result.data.length;
     }
     readCount++;