[dart2wasm] Use identity hashmap for `allowInterop` closure map

This significantly speeds up certain operations as it avoids calling
complex closure equality logic in the hashmap operations.

See also [0] regarding us leaking those closures.

[0] https://github.com/dart-lang/sdk/issues/54908

Change-Id: I046063884b88fbe53bc3cb397b0087693c9b928a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/352520
Reviewed-by: Srujan Gaddam <srujzs@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>

sdk/lib/_internal/wasm/lib/js_helper.dart

diff --git a/sdk/lib/_internal/wasm/lib/js_helper.dart b/sdk/lib/_internal/wasm/lib/js_helper.dart
index 8f27dee..d2c21b1 100644
--- a/sdk/lib/_internal/wasm/lib/js_helper.dart
+++ b/sdk/lib/_internal/wasm/lib/js_helper.dart
@@ -340,7 +340,7 @@
 // `allowInterop` and to replace them with the wrapped variant when they flow
 // to JS.
 // NOTE: We are not currently replacing functions returned from JS.
-Map<Function, JSValue> functionToJSWrapper = {};
+final Map<Function, JSValue> functionToJSWrapper = Map.identity();
 
 WasmExternRef? jsArrayBufferFromDartByteBuffer(ByteBuffer buffer) {
   ByteData byteData = ByteData.view(buffer);