Type-related fixes

pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/operations.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// This library defines runtime operations on objects used by the code
 /// generator.
 part of dart._runtime;
 
 class InvocationImpl extends Invocation {
   final Symbol memberName;
@@ skipping 777 matching lines @@
 constFn(x) => JS('', '() => x');
 
 runtimeType(obj) {
   // Handle primitives where the method isn't on the object.
   var result = _checkPrimitiveType(obj);
   if (result != null) return wrapType(result);
 
   // Delegate to the (possibly user-defined) method on the object.
   var extension = getExtensionType(obj);
   if (extension != null) {
-    return JS('', '#[dartx.runtimeType]', obj);
+    result = JS('', '#[dartx.runtimeType]', obj);
+    // If extension doesn't override runtimeType, return the extension type.
+    return result ?? wrapType(extension);
   }
   if (JS('bool', 'typeof # == "function"', obj)) {
     return wrapType(getReifiedType(obj));
   }
   return JS('', '#.runtimeType', obj);
 }
 
 /// Implements Dart's interpolated strings as ES2015 tagged template literals.
 ///
 /// For example: dart.str`hello ${name}`
@@ skipping 29 matching lines @@
   if (obj != null && getExtensionType(obj) != null) {
     return JS('', 'dartx.#', name);
   }
 
   // Check for certain names that we can't use in JS
   if (name == 'constructor' || name == 'prototype') {
     name = '+' + name;
   }
   return name;
 }