Resynthesize interfaces 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 756fec6a6a8717a6b4a68f657b9443f85e1d48f3..8d141146ba97c396f4feb27ac5fa2336b46576a1 100644
--- a/pkg/analyzer/lib/src/dart/element/element.dart
+++ b/pkg/analyzer/lib/src/dart/element/element.dart
@@ -746,12 +746,19 @@ class ClassElementImpl extends AbstractClassElementImpl
 
   @override
   List<InterfaceType> get interfaces {
-    if (_unlinkedClass != null && _interfaces == null) {
-      ResynthesizerContext context = enclosingUnit.resynthesizerContext;
-      _interfaces = _unlinkedClass.interfaces
-          .map((EntityRef t) => context.resolveTypeRef(this, t))
-          .where(_isClassInterfaceType)
-          .toList(growable: false);
+    if (_interfaces == null) {
+      if (_kernel != null) {
+        _interfaces = _kernel.implementedTypes
+            .map((k) => enclosingUnit._kernelContext.getInterfaceType(this, k))
+            .toList(growable: false);
+      }
+      if (_unlinkedClass != null) {
+        ResynthesizerContext context = enclosingUnit.resynthesizerContext;
+        _interfaces = _unlinkedClass.interfaces
+            .map((EntityRef t) => context.resolveTypeRef(this, t))
+            .where(_isClassInterfaceType)
+            .toList(growable: false);
+      }
     }
     return _interfaces ?? const <InterfaceType>[];
   }