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

Index: pkg/front_end/lib/src/fasta/scanner/array_based_scanner.dart
diff --git a/pkg/front_end/lib/src/fasta/scanner/array_based_scanner.dart b/pkg/front_end/lib/src/fasta/scanner/array_based_scanner.dart
index e5de58514b2674f814aaa7ef6668a86936a29932..3a44acd575a2408f3fad51f72e92e8cb2a330cc2 100644
--- a/pkg/front_end/lib/src/fasta/scanner/array_based_scanner.dart
+++ b/pkg/front_end/lib/src/fasta/scanner/array_based_scanner.dart
@@ -14,7 +14,11 @@ 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;
 
@@ -132,8 +136,11 @@ abstract class ArrayBasedScanner extends AbstractScanner {
     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);
   }