Fix bug regarding augmenting a field with a field after a non-field augment.

In the commit https://dart-review.googlesource.com/c/sdk/+/446702 these
tests were landed with several known flaws. One of these was the
processing of the chain: field -> augment getter -> augment field,
creating two fields.

This CL fixes the behavior such that augment field genuinely augments
the existing field instead of creating a new field. Test expectations
were correspondingly updated.

Change-Id: I6e3c822441a7fb5d2f94a673ed36d409ee5d03e6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/448226
Reviewed-by: Konstantin Shcheglov <[email protected]>
Reviewed-by: Paul Berry <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: fshcheglov

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

@@ -2680,6 +2680,17 @@ class FieldElementImpl extends PropertyInducingElementImpl
   @override
   ElementKind get kind => ElementKind.FIELD;
 
+  FieldFragmentImpl get lastFragment {
+    var result = _firstFragment;
+    while (true) {
+      if (result.nextFragment case var nextFragment?) {
+        result = nextFragment;
+      } else {
+        return result;
+      }
+    }
+  }
+
   @Deprecated('Use library instead')
   @override
   LibraryElementImpl get library2 => library;

pkg/analyzer/lib/src/summary2/element_builder.dart

@@ -295,12 +295,15 @@ class ElementBuilder {
 
   void _handleInstanceFieldFragment(
     InstanceElementImpl instanceElement,
-    FragmentImpl? lastFieldFragment,
+    FragmentImpl? lastFragment,
     FieldFragmentImpl fieldFragment,
   ) {
     var instanceFragment = fieldFragment.enclosingFragment;
     instanceFragment.addField(fieldFragment);
 
+    var lastFieldElement = _fieldElement(lastFragment);
+    var lastFieldFragment = lastFieldElement?.lastFragment;
+
     if (fieldFragment.isAugmentation &&
         lastFieldFragment is FieldFragmentImpl) {
       lastFieldFragment.addFragment(fieldFragment);