Improve error-recovery from unmatched braces.

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

Index: dart/sdk/lib/_internal/compiler/implementation/scanner/token.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/scanner/token.dart b/dart/sdk/lib/_internal/compiler/implementation/scanner/token.dart
index 46f86d7d76abf3ee7fdbb3ade3203e4b63c0f7d5..0b474d417bde822326dffd1e7b4e4d55ddec8f5f 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/scanner/token.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/scanner/token.dart
@@ -264,6 +264,18 @@ class UnterminatedToken extends ErrorToken {
   int get charCount => endOffset - charOffset;
 }
 
+class UnmatchedToken extends ErrorToken {
+  final BeginGroupToken begin;
+
+  UnmatchedToken(BeginGroupToken begin)
+      : this.begin = begin,
+        super(begin.charOffset);
+
+  String toString() => "UnmatchedToken(${begin.value})";
+
+  String get assertionMessage => "'$begin' isn't closed.";
+}
+
 /**
  * A String-valued token. Represents identifiers, string literals,
  * number literals, comments, and error tokens, using the corresponding