Resynthesize metadata from Kernel, when already supported.

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..e5636cc19a5ce67ad85a104abf2c9278718ec47b 100644
--- a/pkg/analyzer/lib/src/dart/element/element.dart
+++ b/pkg/analyzer/lib/src/dart/element/element.dart
@@ -849,6 +849,10 @@ class ClassElementImpl extends AbstractClassElementImpl
 
   @override
   List<ElementAnnotation> get metadata {
+    if (_kernel != null) {
+      _metadata ??= enclosingUnit._kernelContext
+          .buildAnnotations(enclosingUnit, _kernel.annotations);
+    }
     if (_unlinkedClass != null) {
       return _metadata ??=
           _buildAnnotations(enclosingUnit, _unlinkedClass.annotations);
@@ -4086,6 +4090,10 @@ abstract class ExecutableElementImpl extends ElementImpl
 
   @override
   List<ElementAnnotation> get metadata {
+    if (_kernel != null) {
+      _metadata ??= enclosingUnit._kernelContext
+          .buildAnnotations(enclosingUnit, _kernel.annotations);
+    }
     if (serializedExecutable != null) {
       return _metadata ??=
           _buildAnnotations(enclosingUnit, serializedExecutable.annotations);
@@ -5927,6 +5935,12 @@ abstract class KernelLibraryResynthesizerContext {
   kernel.Library get library;
 
   /**
+   * TODO(scheglov) document
+   */
+  List<ElementAnnotation> buildAnnotations(
+      CompilationUnitElementImpl unit, List<kernel.Expression> annotations);
+
+  /**
    * Build explicit top-level property accessors.
    */
   UnitExplicitTopLevelAccessors buildTopLevelAccessors(
@@ -7636,6 +7650,10 @@ abstract class NonParameterVariableElementImpl extends VariableElementImpl {
 
   @override
   List<ElementAnnotation> get metadata {
+    if (_kernel != null) {
+      _metadata ??= enclosingUnit._kernelContext
+          .buildAnnotations(enclosingUnit, _kernel.annotations);
+    }
     if (_unlinkedVariable != null) {
       return _metadata ??=
           _buildAnnotations(enclosingUnit, _unlinkedVariable.annotations);