Add Member.documentationComment and use it to resynthesize documentation from Kernel.

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

Index: pkg/analyzer/lib/src/dart/element/element.dart
diff --git a/pkg/analyzer/lib/src/dart/element/element.dart b/pkg/analyzer/lib/src/dart/element/element.dart
index 5e4dd8cf752a5b8bb9a4d77c61de513ac018c146..8021fb7f11d492a0da398c7f817626bfa1673afd 100644
--- a/pkg/analyzer/lib/src/dart/element/element.dart
+++ b/pkg/analyzer/lib/src/dart/element/element.dart
@@ -3965,6 +3965,9 @@ abstract class ExecutableElementImpl extends ElementImpl
 
   @override
   String get documentationComment {
+    if (_kernel != null) {
+      return _kernel.documentationComment;
+    }
     if (serializedExecutable != null) {
       return serializedExecutable?.documentationComment?.text;
     }
@@ -7556,6 +7559,9 @@ abstract class NonParameterVariableElementImpl extends VariableElementImpl {
 
   @override
   String get documentationComment {
+    if (_kernel != null) {
+      return _kernel.documentationComment;
+    }
     if (_unlinkedVariable != null) {
       return _unlinkedVariable?.documentationComment?.text;
     }