Add 'external-effect' pragma support to all the backends.

Call sites targeting a procedure annotated with `external-effect` will
not produce any code, including the argument which will not be
evaluated.

However, the single parameter will be treated as 'live' for the purposes
of any global analysis the backends do. This is useful for things like
protobuf shaking where a user may want to retain certain protobuf
messages without actually emitting the code that retains those messages.

Today this functionality is available internally in the vm and wasm SDK
libraries. dart2js has similar functionality represented via the
opaqueTrue and opaqueFalse booleans (which will cause conditional
branches to get shaken after analysis). This will replace dart2js's
opaque(True/False).

This also adds validation to the frontend to ensure a method annotated
with 'external-effect' is well-formed.

Change-Id: If1c4096673e655c58fe7638840a16125003e7809
Tested: Backend tests for codegen were added. A frontend test was added for the validation. A language test was added to confirm the behavior.
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/476020
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Nate Biggs <natebiggs@google.com>

tests/language/method/external_effect_test.dart

@@ -0,0 +1,35 @@
+// Copyright (c) 2026, the Dart project authors.  Please see the AUTHORS file
+// 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.
+
+import "package:expect/expect.dart";
+
+@pragma('external-effect')
+external void externalEffect(Object? o);
+
+void noExternalEffect(Object? o) {}
+
+List<Object> used = [];
+
+Null use(int o) {
+  used.add(o);
+  return null;
+}
+
+const Object constObj = Object();
+
+Null useConstObject() {
+  used.add(constObj);
+  return null;
+}
+
+void main() {
+  externalEffect(use(3));
+  externalEffect(useConstObject());
+  Expect.isTrue(used.isEmpty);
+  noExternalEffect(use(4));
+  noExternalEffect(useConstObject());
+  Expect.equals(used.length, 2);
+  Expect.equals(used[0], 4);
+  Expect.equals(used[1], constObj);
+}