Fix crashes in scanner.

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

Index: dart/sdk/lib/_internal/compiler/implementation/scanner/parser.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/scanner/parser.dart b/dart/sdk/lib/_internal/compiler/implementation/scanner/parser.dart
index af56bf995fc049f84f205099015ccc4f17dc0fba..8fcb14683ca9b20fa87fbc890816dcefd7513882 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/scanner/parser.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/scanner/parser.dart
@@ -1778,8 +1778,11 @@ class Parser {
   }
 
   Token parseParenthesizedExpression(Token token) {
-    var begin = (token as BeginGroupToken);
+    // We expect [begin] to be of type [BeginGroupToken], but we don't know for
+    // sure until after calling expect.
+    var begin = token;
     token = expect('(', token);
+    // [begin] is now known to have type [BeginGroupToken].
     token = parseExpression(token);
     if (!identical(begin.endGroup, token)) {
       listener.unexpected(token);