[dart2wasm] Fix missing local names in wasm serializer

There was a missing increment of an integer for the local names to be
written out in the [0] refactoring.

Once we have the wasm module reader checked-in, I could even write a
regression test for it.

[0] https://dart-review.googlesource.com/c/sdk/+/452040

Change-Id: I65367b6796a50f16db1c013c94bb1d413691e980
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/452600
Reviewed-by: Ömer Ağacan <[email protected]>
Commit-Queue: Martin Kustermann <[email protected]>

Author: mkustermann

pkg/wasm_builder/lib/src/serialize/sections.dart

@@ -453,6 +453,7 @@ class NameSection extends CustomSection {
             localNames.writeUnsigned(entry.key);
             localNames.writeName(entry.value);
           }
+          functionsWithLocalNamesCount++;
         }
       }
     }