Completion. Speed. Use identical() in isSubtypeOf().

Before:
[1710693275292][time: 774 ms][mean: 750.0][stdDev: 23.531][min: 693.0][max: 814.0]

After:
[1710696094848][time: 659 ms][mean: 607.0][stdDev: 28.995][min: 548.0][max: 736.0]

So, about 20% faster.

When
```dart
import 'package:flutter/widgets.dart';
Widget x = ^;
```

Change-Id: I30682382748e3e03e9fe28da6cd854280c926b32
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/358100
Reviewed-by: Keerti Parthasarathy <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov

pkg/analyzer/lib/src/dart/element/subtype.dart

@@ -447,7 +447,7 @@ class SubtypeHelper {
 
     var subElement = subType.element;
     var superElement = superType.element;
-    if (subElement == superElement) {
+    if (identical(subElement, superElement)) {
       return _interfaceArguments(superElement, subType, superType);
     }
 
@@ -457,7 +457,7 @@ class SubtypeHelper {
     }
 
     for (var interface in subElement.allSupertypes) {
-      if (interface.element == superElement) {
+      if (identical(interface.element, superElement)) {
         var substitution = Substitution.fromInterfaceType(subType);
         var substitutedInterface =
             substitution.substituteType(interface) as InterfaceType;