Constrain type annotations with deferred types.

sdk/lib/_internal/compiler/implementation/elements/modelx.dart

Index: sdk/lib/_internal/compiler/implementation/elements/modelx.dart
diff --git a/sdk/lib/_internal/compiler/implementation/elements/modelx.dart b/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
index 9a226bced08a8f416c977aaca4b04923f261fcf1..f8e06d245997200c572e46783ddf5a8ef6fa184e 100644
--- a/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
+++ b/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
@@ -1016,6 +1016,8 @@ class PrefixElementX extends ElementX implements PrefixElement {
 
   final ImportScope importScope = new ImportScope();
 
+  bool isDeferred = false;
+
   PrefixElementX(String prefix, Element enclosing, this.firstPosition)
       : super(prefix, ElementKind.PREFIX, enclosing);
 
@@ -1030,6 +1032,10 @@ class PrefixElementX extends ElementX implements PrefixElement {
   }
 
   accept(ElementVisitor visitor) => visitor.visitPrefixElement(this);
+
+  void markAsDeferred() {
+    isDeferred = true;
+  }
 }
 
 class TypedefElementX extends ElementX implements TypedefElement {