Parse argument definition test expressions.

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

Index: dart/lib/compiler/implementation/scanner/parser.dart
diff --git a/dart/lib/compiler/implementation/scanner/parser.dart b/dart/lib/compiler/implementation/scanner/parser.dart
index adf0646b577c3584f77ad01c1738e11950c09c71..a6e09b7e36b3da88d2e3038602495e60ab6c61cf 100644
--- a/dart/lib/compiler/implementation/scanner/parser.dart
+++ b/dart/lib/compiler/implementation/scanner/parser.dart
@@ -1206,11 +1206,22 @@ class Parser {
                (kind === OPEN_CURLY_BRACKET_TOKEN) ||
                token.stringValue === '[]') {
       return parseLiteralListOrMap(token);
+    } else if (kind === QUESTION_TOKEN) {
+      return parseArgumentDefinitionTest(token);
     } else {
       return listener.expectedExpression(token);
     }
   }
 
+  Token parseArgumentDefinitionTest(Token token) {
+    Token questionToken = token;
+    listener.beginArgumentDefinitionTest(questionToken);
+    assert(optional('?', token));
+    token = parseIdentifier(token.next);
+    listener.endArgumentDefinitionTest(questionToken, token);
+    return token;
+  }
+
   Token parseParenthesizedExpressionOrFunctionLiteral(Token token) {
     BeginGroupToken beginGroup = token;
     int kind = beginGroup.endGroup.next.kind;