Fix parser in case of unmatched angle bracket.

sdk/lib/_internal/compiler/implementation/scanner/parser.dart

Index: sdk/lib/_internal/compiler/implementation/scanner/parser.dart
diff --git a/sdk/lib/_internal/compiler/implementation/scanner/parser.dart b/sdk/lib/_internal/compiler/implementation/scanner/parser.dart
index 2a6edb6f9ff5a209547f37c190e85752ca9d2deb..7708cac7bb33bcfcaa409aa9fa43baa4930667dd 100644
--- a/sdk/lib/_internal/compiler/implementation/scanner/parser.dart
+++ b/sdk/lib/_internal/compiler/implementation/scanner/parser.dart
@@ -700,6 +700,9 @@ class Parser {
         if (optional('<', token.next)) {
           if (token.next is BeginGroupToken) {
             BeginGroupToken beginGroup = token.next;
+            if (beginGroup.endGroup == null) {
+              listener.unmatched(beginGroup);
+            }
             token = beginGroup.endGroup;
           }
         }