[dart2wasm] Return null in void typed interop functions

Instead of boxing the return values just return `null`.

Also drop the result type from the imports when the return type is
`void`.

Change-Id: I871717480e516ac4cec260b2f9f4abe3dd5df535
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/424742
Commit-Queue: Ömer Ağacan <[email protected]>
Reviewed-by: Martin Kustermann <[email protected]>

Author: osa1

pkg/dart2wasm/lib/js/interop_specializer.dart

@@ -100,7 +100,9 @@ abstract class _Specializer {
         'dart2wasm.$jsMethodName',
         FunctionNode(null,
             positionalParameters: dartPositionalParameters,
-            returnType: _util.nullableWasmExternRefType),
+            returnType: function.returnType is VoidType
+                ? VoidType()
+                : _util.nullableWasmExternRefType),
         fileUri,
         AnnotationType.import,
         isExternal: true);

pkg/dart2wasm/lib/js/util.dart

@@ -326,14 +326,17 @@ class CoreTypesUtil {
   Expression castInvocationForReturn(
       Expression invocation, DartType returnType) {
     Expression expression;
-    if (returnType is VoidType || isJSValueType(returnType)) {
+    if (returnType is VoidType) {
       // Technically a `void` return value can still be used, by casting the
       // return type to `dynamic` or `Object?`. However this case should be
-      // extremely rare, and `dartifyRaw` overhead for return values that will
+      // extremely rare, and `dartifyRaw` overhead for return values that should
       // never be used in practice is too much, so we avoid `dartifyRaw` on
-      // `void` returns. We still box the `externref` as the value can be passed
-      // around as a Dart object.
+      // `void` returns and always return `null`.
+      return BlockExpression(
+          Block([ExpressionStatement(invocation)]), NullLiteral());
+    }
 
+    if (isJSValueType(returnType)) {
       // TODO(joshualitt): Expose boxed `JSNull` and `JSUndefined` to Dart
       // code after migrating existing users of js interop on Dart2Wasm.
       // expression = _createJSValue(invocation);