Parse HTML files with keeping original attributes case.

pkg/analyzer/lib/src/task/html.dart

Index: pkg/analyzer/lib/src/task/html.dart
diff --git a/pkg/analyzer/lib/src/task/html.dart b/pkg/analyzer/lib/src/task/html.dart
index d8930b79da2add514ce3b7a1894458be952fb946..b6a771a8651c8229efa0892d1b4580f53fceeeb5 100644
--- a/pkg/analyzer/lib/src/task/html.dart
+++ b/pkg/analyzer/lib/src/task/html.dart
@@ -313,7 +313,8 @@ class ParseHtmlTask extends SourceBasedAnalysisTask {
       ];
       outputs[LINE_INFO] = new LineInfo(<int>[0]);
     } else {
-      HtmlParser parser = new HtmlParser(content, generateSpans: true);
+      HtmlParser parser = new HtmlParser(content,
+          generateSpans: true, lowercaseAttrName: false);
       parser.compatMode = 'quirks';
       Document document = parser.parse();
       //