[dart2wasm] Use unchecked entry if an InstanceInvocation has isInvariant flag set

The CFE marks nodes as invariant if it's guaranteed the covariance
checks would succeed. It does so e.g. in this example

  final list = <int>[];
  list.add(1);

Change-Id: I2c9b0883e0c69e128c8014934a325ce8ef21a8d0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/416942
Commit-Queue: Martin Kustermann <[email protected]>
Reviewed-by: Ömer Ağacan <[email protected]>

Author: mkustermann

pkg/dart2wasm/lib/translator.dart

@@ -1440,6 +1440,7 @@ class Translator with KernelNodes {
 
   bool canUseUncheckedEntry(Expression receiver, Expression node) {
     if (receiver is ThisExpression) return true;
+    if (node is InstanceInvocation && node.isInvariant) return true;
     return inferredTypeMetadata[node]?.skipCheck ?? false;
   }