[dart2wasm] Add _Function.call member

New passing test: language/call/method_function_typed_value_test/08

Change-Id: If959ce05c48658407011044f5ff3df9076f04bdc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/276900
Commit-Queue: Ömer Ağacan <omersa@google.com>
Reviewed-by: Aske Simon Christensen <askesc@google.com>

sdk/lib/_internal/wasm/lib/function.dart

@@ -25,4 +25,7 @@ class _Function implements Function {
   // Simple hash code for now, we can optimize later
   @override
   int get hashCode => runtimeType.hashCode;
+
+  // Support dynamic tear-off of `.call` on functions
+  _Function get call => this;
 }