[dart2wasm] Override runtime types of internal types

Runtime types of String, int, and double implementations are expected to
be `String`, `int`, and `double`, respectively.

Change-Id: I8f84ee2e8405c820a686d54449d3eb2db18eb52a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/276643
Commit-Queue: Ömer Ağacan <omersa@google.com>
Reviewed-by: Aske Simon Christensen <askesc@google.com>

sdk/lib/_internal/wasm/lib/double.dart

@@ -63,6 +63,9 @@ class _BoxedDouble extends double {
   /// Dummy factory to silence error about missing superclass constructor.
   external factory _BoxedDouble();
 
+  @override
+  Type get runtimeType => double;
+
   static const int _mantissaBits = 52;
   static const int _exponentBits = 11;
   static const int _exponentBias = 1023;

sdk/lib/_internal/wasm/lib/int.dart

@@ -31,6 +31,9 @@ class _BoxedInt extends int {
   /// Dummy factory to silence error about missing superclass constructor.
   external factory _BoxedInt();
 
+  @override
+  Type get runtimeType => int;
+
   external num operator +(num other);
   external num operator -(num other);
   external num operator *(num other);

sdk/lib/_internal/wasm/lib/string_patch.dart

@@ -95,6 +95,9 @@ abstract class _StringBase implements String {
 
   _StringBase._();
 
+  @override
+  Type get runtimeType => String;
+
   int get hashCode {
     int hash = _getHash(this);
     if (hash != 0) return hash;