Introduce DartType.unaliased to avoid use of Resolution in the backend.

pkg/compiler/lib/src/elements/modelx.dart

Index: pkg/compiler/lib/src/elements/modelx.dart
diff --git a/pkg/compiler/lib/src/elements/modelx.dart b/pkg/compiler/lib/src/elements/modelx.dart
index 488c7e11de99d809ba9f744c350b5f5bcc21c4d8..5cb0fe201d2a79407f9a3bb6e551e53aee494eb7 100644
--- a/pkg/compiler/lib/src/elements/modelx.dart
+++ b/pkg/compiler/lib/src/elements/modelx.dart
@@ -1318,7 +1318,13 @@ class TypedefElementX extends ElementX
   /**
    * The type annotation which defines this typedef.
    */
-  DartType alias;
+  DartType aliasCache;
+
+  DartType get alias {
+    assert(invariant(this, hasBeenCheckedForCycles,
+        message: "$this has not been checked for cycles."));
+    return aliasCache;
+  }
 
   /// [:true:] if the typedef has been checked for cyclic reference.
   bool hasBeenCheckedForCycles = false;