[dart2wasm] Fix popWasmArray positional arguments assert.

popWasmArray is called with 2 arguments, not 4:
https://github.com/dart-lang/sdk/blob/main/sdk/lib/_internal/wasm/lib/convert_patch.dart#L103

Change-Id: I848a26d7213af88cf070184ef139bd26010d28bd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/381560
Commit-Queue: Ömer Ağacan <omersa@google.com>
Reviewed-by: Ömer Ağacan <omersa@google.com>

pkg/dart2wasm/lib/transformers.dart

@@ -929,7 +929,7 @@ class PushPopWasmArrayTransformer {
         elementType.withDeclaredNullability(Nullability.nullable);
 
     final positionalArguments = invocation.arguments.positional;
-    assert(positionalArguments.length == 4);
+    assert(positionalArguments.length == 2);
 
     final array = positionalArguments[0];
     final length = positionalArguments[1];