Fix breaking change from next SDK

lib/src/analyzer.dart

Index: lib/src/analyzer.dart
diff --git a/lib/src/analyzer.dart b/lib/src/analyzer.dart
index 07e5bd40f035dd36c466138dc9d2a609b1ff5841..5683a570f55910a65efbdbbde1d6a057bc9ef38b 100644
--- a/lib/src/analyzer.dart
+++ b/lib/src/analyzer.dart
@@ -8,7 +8,7 @@
  */
 library analyzer;
 
-import 'package:csslib/parser.dart' as Css;
+import 'package:csslib/parser.dart' as css;
 import 'package:csslib/visitor.dart' show StyleSheet, treeToDebugString, Visitor, Expressions, VarDefinition;
 import 'package:html5lib/dom.dart';
 import 'package:html5lib/dom_parsing.dart';
@@ -1100,7 +1100,7 @@ class _AnalyzerCss {
     var styleSheets = _dependencies(libraryInfo).toList();
 
     var errors = [];
-    Css.analyze(styleSheets, errors: errors, options:
+    css.analyze(styleSheets, errors: errors, options:
       [_warningsAsErrors ? '--warnings_as_errors' : '', 'memory']);
 
     // Print errors as warnings.