Delay 'supertype' for ClassElementImpl and EnumElementImpl.

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 25e5ede2c97e483a82d2dd454f38330f0fce49b3..e42f809d98f76569012509d0e01a3c9f1e5f5eaa 100644
--- a/pkg/analyzer/lib/src/dart/element/element.dart
+++ b/pkg/analyzer/lib/src/dart/element/element.dart
@@ -80,7 +80,7 @@ abstract class AbstractClassElementImpl extends ElementImpl
    * explicit superclass.
    */
   @override
-  InterfaceType supertype;
+  InterfaceType _supertype;
 
   /**
    * The type defined by the class.
@@ -1138,6 +1138,26 @@ class ClassElementImpl extends AbstractClassElementImpl {
   }
 
   @override
+  InterfaceType get supertype {
+    if (_unlinkedClass != null && _supertype == null) {
+      if (_unlinkedClass.supertype != null) {
+        _supertype = enclosingUnit.resynthesizerContext
+            .resolveTypeRef(_unlinkedClass.supertype, this);
+      } else if (_unlinkedClass.hasNoSupertype) {
+        return null;
+      } else {
+        _supertype = context.typeProvider.objectType;
+      }
+    }
+    return _supertype;
+  }
+
+  void set supertype(InterfaceType supertype) {
+    assert(_unlinkedClass == null);
+    _supertype = supertype;
+  }
+
+  @override
   List<TypeParameterElement> get typeParameters {
     if (_unlinkedClass != null) {
       return resynthesizedTypeParameters;
@@ -3056,6 +3076,9 @@ class EnumElementImpl extends AbstractClassElementImpl {
   }
 
   @override
+  InterfaceType get supertype => context.typeProvider.objectType;
+
+  @override
   List<TypeParameterElement> get typeParameters =>
       const <TypeParameterElement>[];