[dart2wasm] Fix invalid closure return type in async* lowering

dart2wasm lowering of async* methods includes call to Stream.where
which takes a closure. The return type of that closure was incorrectly
set to Object?, while it should be bool.

This incorrect static type becomes a problem with more precise
handling of closures in TFA.

Issue: https://github.com/dart-lang/sdk/issues/39692
Change-Id: I35a8a6b198413d564e091a935a8beaff15d6d541
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/339200
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Ömer Ağacan <omersa@google.com>

pkg/dart2wasm/lib/transformers.dart

@@ -590,7 +590,7 @@ class _WasmTransformer extends Transformer {
           ReturnStatement(VariableGet(whereKeep)),
         ]),
         positionalParameters: [whereFilterArg],
-        returnType: coreTypes.objectNullableRawType));
+        returnType: coreTypes.boolNonNullableRawType));
 
     Procedure whereProc =
         coreTypes.index.getProcedure('dart:async', 'Stream', 'where');

pkg/front_end/testcases/dart2wasm/yield.dart.strong.transformed.expect

@@ -104,7 +104,7 @@ static method asyncMethod(asy::Stream<core::int> stream) → asy::Stream<core::i
     if(!#isEven)
       value.{asy::Completer::complete}(#C2){([FutureOr<core::bool>?]) → void};
     return value;
-  }){((core::Object?) → FutureOr<core::Object?>) → asy::Stream<core::Object?>}.{asy::Stream::where}((synthesized core::Object? value) → core::Object? {
+  }){((core::Object?) → FutureOr<core::Object?>) → asy::Stream<core::Object?>}.{asy::Stream::where}((synthesized core::Object? value) → core::bool {
     if(#isFirst) {
       #isFirst = #C1;
       return #C1;