add previous token field

pkg/front_end/test/scanner_fasta_test.dart

Index: pkg/front_end/test/scanner_fasta_test.dart
diff --git a/pkg/front_end/test/scanner_fasta_test.dart b/pkg/front_end/test/scanner_fasta_test.dart
index 0003ba12c6ef0ed6fce9392aba77a31c21d8ae08..14deb22c1ad05a1f7c3b4a3e0b9d46a8ceab4cdc 100644
--- a/pkg/front_end/test/scanner_fasta_test.dart
+++ b/pkg/front_end/test/scanner_fasta_test.dart
@@ -194,6 +194,23 @@ class ScannerTest_Fasta extends ScannerTestBase {
     super.test_scriptTag_withArgs();
   }
 
+  void test_next_previous() {
+    const source = 'int a; /*1*/ /*2*/ /*3*/ B f(){if (a < 2) {}}';
+    fasta.Token token =
+        new fasta.StringScanner(source, includeComments: true).tokenize();
+    while (!token.isEof) {
+      expect(token.next.previousToken, token);
+      fasta.Token commentToken = token.precedingComments;
+      while (commentToken != null) {
+        if (commentToken.next != null) {
+          expect(commentToken.next.previousToken, commentToken);
+        }
+        commentToken = commentToken.next;
+      }
+      token = token.next;
+    }
+  }
+
   @override
   @failingTest
   void test_scriptTag_withoutSpace() {