[dart2wasm] Fix new web/wasm/allow_import_export_pragmas_test test: A…

…dd `--extra-compiler-option=` before the flag name

This wasn't caught by default CI builders, because
default configurations use `pkg/dart2wasm/tool/compile_benchmark`
which passes all unknown flags to the compiler (for convenience
of local development) but `dart compile wasm` doesn't

Change-Id: I6a92d3f04848e027c92b1feee3d54e7efba9fd27
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/371661
Commit-Queue: Martin Kustermann <kustermann@google.com>
Reviewed-by: Ömer Ağacan <omersa@google.com>

tests/web/wasm/allow_import_export_pragmas_test.dart

@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// dart2wasmOptions=--enable-experimental-wasm-interop
+// dart2wasmOptions=--extra-compiler-option=--enable-experimental-wasm-interop
 
 // Test that importing `dart:_wasm` and using import/export pragmas works if the
 // compiler is given the `--enable-experimental-wasm-interop` flag.