improve handling of missing interpolation identifier

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 67f3582c546c195628afee01b191120bb7e9cb62..52c459557b53ed20b6b3569abd93f20e6323b587 100644
--- a/pkg/front_end/test/scanner_fasta_test.dart
+++ b/pkg/front_end/test/scanner_fasta_test.dart
@@ -203,22 +203,6 @@ main() {}
   }
 
   @override
-  @failingTest
-  void test_mismatched_closer() {
-    // TODO(paulberry,ahe): Fasta and analyzer recover this error differently.
-    // Figure out which recovery technique we want the front end to use.
-    super.test_mismatched_closer();
-  }
-
-  @override
-  @failingTest
-  void test_mismatched_opener() {
-    // TODO(paulberry,ahe): Fasta and analyzer recover this error differently.
-    // Figure out which recovery technique we want the front end to use.
-    super.test_mismatched_opener();
-  }
-
-  @override
   void test_mismatched_opener_in_interpolation() {
     // When openers and closers are mismatched,
     // fasta favors considering the opener to be mismatched
@@ -301,6 +285,34 @@ abstract class ScannerTest_Fasta_Base {
     }
   }
 
+  void test_string_simple_interpolation_missingIdentifier() {
+    Token token = scan("'\$x\$'");
+    expectToken(token, TokenType.STRING, 0, 1, lexeme: "'");
+
+    token = token.next;
+    expectToken(token, TokenType.STRING_INTERPOLATION_IDENTIFIER, 1, 1);
+
+    token = token.next;
+    expectToken(token, TokenType.IDENTIFIER, 2, 1, lexeme: 'x');
+
+    token = token.next;
+    expectToken(token, TokenType.STRING, 3, 0, lexeme: '', isSynthetic: true);
+
+    token = token.next;
+    expectToken(token, TokenType.STRING_INTERPOLATION_IDENTIFIER, 3, 1);
+
+    token = token.next;
+    expectToken(token, TokenType.IDENTIFIER, 4, 0,
+        lexeme: '', isSynthetic: true);
+
+    token = token.next;
+    expect((token as fasta.ErrorToken).errorCode,
+        same(codeUnexpectedDollarInString));
+
+    token = token.next;
+    expectToken(token, TokenType.STRING, 4, 1, lexeme: "'");
+  }
+
   void test_string_simple_unterminated_interpolation_block() {
     Token token = scan(r'"foo ${bar');
     expectToken(token, TokenType.STRING, 0, 5, lexeme: '"foo ');
@@ -393,6 +405,10 @@ abstract class ScannerTest_Fasta_Base {
     expectToken(token, TokenType.STRING_INTERPOLATION_IDENTIFIER, 5, 1);
 
     token = token.next;
+    expectToken(token, TokenType.IDENTIFIER, 6, 0,
+        isSynthetic: true, lexeme: '');
+
+    token = token.next;
     expect((token as fasta.ErrorToken).errorCode,
         same(codeUnexpectedDollarInString));
     expect((token as fasta.UnterminatedToken).start, r'$');