Add experimental warning to dart2wasm

Change-Id: I81f562e9a003e7530120fec608baf5984f3a224f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/233783
Reviewed-by: Aske Simon Christensen <askesc@google.com>
Commit-Queue: Michael Thomsen <mit@google.com>
diff --git a/pkg/dart2wasm/bin/dart2wasm.dart b/pkg/dart2wasm/bin/dart2wasm.dart
index 10f1750..19b9604 100644
--- a/pkg/dart2wasm/bin/dart2wasm.dart
+++ b/pkg/dart2wasm/bin/dart2wasm.dart
@@ -33,6 +33,9 @@
 Never usage(String message) {
   print("Usage: dart2wasm [<options>] <infile.dart> <outfile.wasm>");
   print("");
+  print("*NOTE*: Wasm compilation is experimental.");
+  print("The support may change, or be removed, with no advance notice.");
+  print("");
   print("Options:");
   print("  --dart-sdk=<path>");
   print("");