Fix "extends" checks. Move them out of resolve phase and do them on demand.

frog/element.dart

Index: frog/element.dart
diff --git a/frog/element.dart b/frog/element.dart
index 8efe2207d861bc0f4d98c26ab6a74f666d73bf78..7cd199af5dbc1d4222976294dd0a9ecf8ce77cbf 100644
--- a/frog/element.dart
+++ b/frog/element.dart
@@ -103,14 +103,7 @@ class Element implements Hashable {
       }
       var typeArgs = [];
       for (int i=0; i < typeRef.typeArguments.length; i++) {
-        var extendsType = baseType.typeParameters[i].extendsType;
-        var typeArg = resolveType(typeRef.typeArguments[i], typeErrors);
-        typeArgs.add(typeArg);
-
-        if (extendsType != null && typeArg is! ParameterType) {
-          typeArg.ensureSubtypeOf(extendsType,
-              typeRef.typeArguments[i].span, typeErrors);
-        }
+        typeArgs.add(resolveType(typeRef.typeArguments[i], typeErrors));
       }
       typeRef.type = baseType.getOrMakeConcreteType(typeArgs);
     } else if (node is FunctionTypeReference) {