[dartdevc] Hoisting subtype caches to addTypeCaches.

This lets us avoid some runtime polymorphicity of JS objects.

Contains a subset of optimizations in https://dart-review.googlesource.com/c/sdk/+/136845

Change-Id: I6cdcf6c58a51074ad860ea225db512f28fff5436
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/138564
Reviewed-by: Sigmund Cherem <[email protected]>
Reviewed-by: Nicholas Shahan <[email protected]>

Author: Markzipan

sdk_nnbd/lib/_internal/js_dev_runtime/private/ddc_runtime/classes.dart

@@ -573,6 +573,9 @@ addTypeTests(ctor, isClass) {
 addTypeCaches(type) {
   JS('', '#[#] = void 0', type, _cachedLegacy);
   JS('', '#[#] = void 0', type, _cachedNullable);
+  var subtypeCacheMap = JS<Object>('!', 'new Map()');
+  JS('', '#[#] = #', type, _subtypeCache, subtypeCacheMap);
+  JS('', '#.push(#)', _cacheMaps, subtypeCacheMap);
 }
 
 // TODO(jmesserly): should we do this for all interfaces?

sdk_nnbd/lib/_internal/js_dev_runtime/private/ddc_runtime/types.dart

@@ -273,6 +273,9 @@ final _cachedNullable = JS('', 'Symbol("cachedNullable")');
 /// A javascript Symbol used to store a canonical version of T* on T.
 final _cachedLegacy = JS('', 'Symbol("cachedLegacy")');
 
+/// A javascript Symbol used to store prior subtype checks and their results.
+final _subtypeCache = JS('', 'Symbol("_subtypeCache")');
+
 /// Returns a nullable (question, ?) version of [type].
 ///
 /// The resulting type returned in a normalized form based on the rules from the
@@ -1235,17 +1238,9 @@ _isFunctionSubtype(ft1, ft2, bool strictMode) => JS('', '''(() => {
 bool isSubtypeOf(Object t1, Object t2) {
   // TODO(jmesserly): we've optimized `is`/`as`/implicit type checks, so they're
   // dispatched on the type. Can we optimize the subtype relation too?
-  // TODO: Find a way to eagerly attach this cache to the Null object at
-  // compile-time so we can remove the top-level null comparison cache entirely.
-  Object map;
-  if (JS('!', '!#.hasOwnProperty(#)', t1, _subtypeCache)) {
-    JS('', '#[#] = #', t1, _subtypeCache, map = JS<Object>('!', 'new Map()'));
-    _cacheMaps.add(map);
-  } else {
-    map = JS<Object>('!', '#[#]', t1, _subtypeCache);
-    bool result = JS('', '#.get(#)', map, t2);
-    if (JS('!', '# !== void 0', result)) return result;
-  }
+  var map = JS<Object>('!', '#[#]', t1, _subtypeCache);
+  bool result = JS('', '#.get(#)', map, t2);
+  if (JS('!', '# !== void 0', result)) return result;
 
   var validSubtype = _isSubtype(t1, t2, true);
   if (!validSubtype && !_strictSubtypeChecks) {
@@ -1261,8 +1256,6 @@ bool isSubtypeOf(Object t1, Object t2) {
   return validSubtype;
 }
 
-final _subtypeCache = JS('', 'Symbol("_subtypeCache")');
-
 @notNull
 bool _isBottom(type, @notNull bool strictMode) =>
     _equalType(type, Never) || (!strictMode && _equalType(type, Null));