[dart2wasm] Use String as return type of Record_<X>.toString()

This will ensure selector computation for `toString()` selector will
use more precise type.

Change-Id: I668197e58d25ac225fd8428dfaa5093e9090de2e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/431041
Reviewed-by: Slava Egorov <vegorov@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>

pkg/dart2wasm/lib/record_class_generator.dart

@@ -395,7 +395,8 @@ class _RecordClassGenerator {
         Procedure(
           Name('toString', library),
           ProcedureKind.Method,
-          FunctionNode(ReturnStatement(stringExpression)),
+          FunctionNode(ReturnStatement(stringExpression),
+              returnType: coreTypes.stringNonNullableRawType),
           fileUri: library.fileUri,
         ),
         coreTypes);