Remove duplicate keys in dartfuzz_api_table

Duplications introduced in
cfa03ed

Change-Id: I9720822611aacefb9b1538d9fc59e234f96475bf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/321842
Auto-Submit: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>

runtime/tools/dartfuzz/gen_api_table.dart

@@ -221,12 +221,11 @@ final Map<String, String> typeToLibraryMethodsListName = () {
     unsupportedErrorEncoding: unsupportedErrorLibs,
     voidEncoding: voidLibs,
   };
-  var result = <String, String>{};
-  encodings.forEach((key, value) {
-    result[key] = value;
-    result[key + "_NULLABLE"] = value + "Nullable";
-  });
-  return result;
+  return {
+    for (var MapEntry(:key, :value) in encodings.entries) key: value,
+    for (var MapEntry(:key, :value) in encodings.entries)
+      if (key != voidEncoding) '${key}_NULLABLE': '${value}Nullable',
+  };
 }();
 
 // Map from return type encoding to list of recognized methods with that