[ddc] Cleanup old setBaseClass()

With the new type system type arguments appearing in extends are
no longer represented by a class definition and do not need to
trigger any lazy logic.

Change-Id: I3f80f9f972bec69c678909c12d25a604a8fe2c59
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/353205
Commit-Queue: Nicholas Shahan <nshahan@google.com>
Reviewed-by: Mark Zhou <markzipan@google.com>

pkg/dev_compiler/lib/src/kernel/compiler.dart

@@ -1265,19 +1265,9 @@ class ProgramCompiler extends ComputeOnceConstantVisitor<js_ast.Expression>
     _classEmittingExtends = c;
 
     // Unroll mixins.
-    if (shouldDefer(supertype)) {
-      var originalSupertype = supertype;
-      deferredSupertypes.add(() => runtimeStatement('setBaseClass(#, #)', [
-            getBaseClass(mixinApplications.length),
-            emitDeferredClassRef(originalSupertype),
-          ]));
-      // Refers to 'supertype' without type parameters. We remove these from
-      // the 'extends' clause for generics for cyclic dependencies and append
-      // them later with 'setBaseClass'.
-      supertype =
-          _coreTypes.rawType(supertype.classNode, _currentLibrary!.nonNullable);
-    }
-    var baseClass = emitClassRef(supertype);
+    var baseClass = shouldDefer(supertype)
+        ? emitDeferredClassRef(supertype)
+        : emitClassRef(supertype);
 
     // TODO(jmesserly): we need to unroll kernel mixins because the synthetic
     // classes lack required synthetic members, such as constructors.

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

@@ -531,15 +531,9 @@ definePrimitiveHashCode(proto) {
       getOwnPropertyDescriptor(proto, extensionSymbol('hashCode')));
 }
 
-/// Link the extension to the type it's extending as a base class.
-void setBaseClass(@notNull Object derived, @notNull Object base) {
-  jsObjectSetPrototypeOf(
-      JS('', '#.prototype', derived), JS('', '#.prototype', base));
-  // We use __proto__ to track the superclass hierarchy (see isSubtypeOf).
-  jsObjectSetPrototypeOf(derived, base);
-}
-
-/// Like [setBaseClass], but for generic extension types such as `JSArray<E>`.
+/// Link the [dartType] to the native [jsType] it is extending as a base class.
+///
+/// Used for generic extension types such as `JSArray<E>`.
 void setExtensionBaseClass(@notNull Object dartType, @notNull Object jsType) {
   // Mark the generic type as an extension type and link the prototype objects.
   var dartProto = JS<Object>('!', '#.prototype', dartType);