Improve parsing of async and generator methods.

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

Index: pkg/front_end/lib/src/fasta/scanner/token.dart
diff --git a/pkg/front_end/lib/src/fasta/scanner/token.dart b/pkg/front_end/lib/src/fasta/scanner/token.dart
index 5d722862f788de27f9c9fbd92fa546ed8e42f1f8..c162ee6443df24b65ff1b64857b6b437f2c19c38 100644
--- a/pkg/front_end/lib/src/fasta/scanner/token.dart
+++ b/pkg/front_end/lib/src/fasta/scanner/token.dart
@@ -96,6 +96,8 @@ abstract class Token {
    */
   bool isIdentifier();
 
+  bool get isPseudo => false;
+
   /**
    * Returns a textual representation of this token to be used for debugging
    * purposes. The resulting string might contain information about the
@@ -178,6 +180,8 @@ class KeywordToken extends Token {
 
   bool isIdentifier() => keyword.isPseudo || keyword.isBuiltIn;
 
+  bool get isPseudo => keyword.isPseudo;
+
   bool get isBuiltInIdentifier {
     // TODO(ahe): Remove special case for "deferred" once dartbug.com/29069 is
     // fixed.