Do not register typechecks against type variables of typedefs.

sdk/lib/_internal/compiler/implementation/dart_types.dart

Index: sdk/lib/_internal/compiler/implementation/dart_types.dart
diff --git a/sdk/lib/_internal/compiler/implementation/dart_types.dart b/sdk/lib/_internal/compiler/implementation/dart_types.dart
index ec79917d6858c80dbf36c8320d8d9f55d2e20c33..a289a58c52bd2c25688d9dafc26ff1730b0690e2 100644
--- a/sdk/lib/_internal/compiler/implementation/dart_types.dart
+++ b/sdk/lib/_internal/compiler/implementation/dart_types.dart
@@ -994,6 +994,10 @@ class Types {
     return subtypeVisitor.isAssignable(r, s);
   }
 
+  static bool isTypeVariableOfTypedef(DartType type) {
+    return type.kind == TypeKind.TYPE_VARIABLE &&
+        type.element.enclosingElement.isTypedef();
+  }
 
   /**
    * Helper method for performing substitution of a linked list of types.