Resynthesize _DeferredClassElement instead of actual ClassElementImpl.

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 585a012336957e4726023786b012245b00b9451d..e458c33d83996e9de0d8dae0875f33c3f36d5840 100644
--- a/pkg/analyzer/lib/src/dart/element/element.dart
+++ b/pkg/analyzer/lib/src/dart/element/element.dart
@@ -1082,7 +1082,9 @@ class CompilationUnitElementImpl extends UriReferencedElementImpl
    */
   void set types(List<ClassElement> types) {
     for (ClassElement type in types) {
-      (type as ClassElementImpl).enclosingElement = this;
+      if (type is ClassElementImpl) {

[Paul Berry, 2016/03/24 19:40:14]

Nit: it's not obvious without reading the rest of the CL why this change is
necessary, which makes me worry that when someone comes back to maintain this
code later they'll be confused.

Consider changing to "if (type is! DeferredClassElement)"

so that if someone comes across this code later they can just navigate to
DeferredClassElement and figure out what is going on.

+        type.enclosingElement = this;
+      }
     }
     this._types = types;
   }