Re-land "Add support for metadata on type variables to Fasta parser." (with fixes).

pkg/front_end/lib/src/fasta/scanner/array_based_scanner.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.scanner.array_based_scanner;
 
 import 'error_token.dart' show ErrorToken;
 
 import 'keyword.dart' show Keyword;
 
 import 'precedence.dart' show COMMENT_INFO, EOF_INFO, PrecedenceInfo;
 
 import 'token.dart'
     show BeginGroupToken, KeywordToken, StringToken, SymbolToken, Token;
 
 import 'token_constants.dart'
-    show LT_TOKEN, OPEN_CURLY_BRACKET_TOKEN, STRING_INTERPOLATION_TOKEN;
+    show
+        LT_TOKEN,
+        OPEN_CURLY_BRACKET_TOKEN,
+        OPEN_PAREN_TOKEN,
+        STRING_INTERPOLATION_TOKEN;
 
 import 'characters.dart' show $LF, $STX;
 
 import 'abstract_scanner.dart' show AbstractScanner;
 
 import '../util/link.dart' show Link;
 
 abstract class ArrayBasedScanner extends AbstractScanner {
   bool hasErrors = false;
 
@@ skipping 97 matching lines @@
   }
 
   /**
    * Appends a token that begins a new group, represented by [info].
    * Group begin tokens are '{', '(', '[' and '${'.
    */
   void appendBeginGroup(PrecedenceInfo info) {
     Token token = new BeginGroupToken(info, tokenStart);
     appendToken(token);
 
-    // { (  [ ${ cannot appear inside a type parameters / arguments.
-    if (!identical(info.kind, LT_TOKEN)) discardOpenLt();
+    // { [ ${ cannot appear inside a type parameters / arguments.
+    if (!identical(info.kind, LT_TOKEN) &&
+        !identical(info.kind, OPEN_PAREN_TOKEN)) {
+      discardOpenLt();
+    }
     groupingStack = groupingStack.prepend(token);
   }
 
   /**
    * Appends a token that begins an end group, represented by [info].
    * It handles the group end tokens '}', ')' and ']'. The tokens '>' and
    * '>>' are handled separately bo [appendGt] and [appendGtGt].
    */
   int appendEndGroup(PrecedenceInfo info, int openKind) {
     assert(!identical(openKind, LT_TOKEN)); // openKind is < for > and >>
@@ skipping 118 matching lines @@
    * something which cannot possibly be part of a type parameter/argument
    * list, like the '=' in the above example.
    */
   void discardOpenLt() {
     while (!groupingStack.isEmpty &&
         identical(groupingStack.head.kind, LT_TOKEN)) {
       groupingStack = groupingStack.tail;
     }
   }
 }