Diagnose unbalanced parentheses better.

dart/lib/compiler/implementation/scanner/string_scanner.dart

Index: dart/lib/compiler/implementation/scanner/string_scanner.dart
diff --git a/dart/lib/compiler/implementation/scanner/string_scanner.dart b/dart/lib/compiler/implementation/scanner/string_scanner.dart
index bdd913107d1d9da977d3e74f6b168c41286bb962..34acd7f085ed77e09d7d3fa97b58d6e3e6c5b286 100644
--- a/dart/lib/compiler/implementation/scanner/string_scanner.dart
+++ b/dart/lib/compiler/implementation/scanner/string_scanner.dart
@@ -32,6 +32,23 @@ class StringScanner extends ArrayBasedScanner<SourceString> {
     tail.next = new StringToken.fromSource(info, value, tokenStart);
     tail = tail.next;
   }
+
+  void unmatchedBeginGroup(BeginGroupToken begin) {
+    SourceString error = new SourceString('unmatched "${begin.stringValue}"');
+    Token close =
+        new StringToken.fromSource(BAD_INPUT_INFO, error, begin.charOffset);
+    // We want to ensure that unmatched BeginGroupTokens are reported
+    // as errors. However, the rest of the parser assume the groups
+    // are well-balanced and will never look at the endGroup
+    // token. This is a nice property that allows us to skip quickly
+    // over correct code. By inserting an additional error token in
+    // the stream, we can keep ignoring endGroup tokens.
+    Token next =
+        new StringToken.fromSource(BAD_INPUT_INFO, error, begin.charOffset);
+    begin.endGroup = close;
+    close.next = next;
+    next.next = begin.next;
+  }
 }
 
 class SubstringWrapper implements SourceString {