Use limited JavaStringBuilder implementation instead of Dart StringBuffer.

pkg/analyzer-experimental/lib/src/generated/html_scanner.dart

Index: pkg/analyzer-experimental/lib/src/generated/html_scanner.dart
diff --git a/pkg/analyzer-experimental/lib/src/generated/html_scanner.dart b/pkg/analyzer-experimental/lib/src/generated/html_scanner.dart
index fa9a3c176ac20716c638e3cda36b48739a123c6f..c09158826b711c94a783f0f8d33e12e93952583e 100644
--- a/pkg/analyzer-experimental/lib/src/generated/html_scanner.dart
+++ b/pkg/analyzer-experimental/lib/src/generated/html_scanner.dart
@@ -1110,9 +1110,9 @@ class XmlTagNode extends XmlNode {
     if (identical(token, _contentEnd)) {
       return content;
     }
-    StringBuffer buffer = new StringBuffer();
+    JavaStringBuilder buffer = new JavaStringBuilder();
     while (token != _contentEnd) {
-      buffer.write(token.lexeme);
+      buffer.append(token.lexeme);
       token = token.next;
     }
     return buffer.toString();