Parser: fix error reporting when pattern assignment declares a variable without a name.

The analyzer/CFE shared error template framework requires name
substitutions to be non-empty. However, sometimes, due to parser error
recovery, an identifier token can be an empty string.

One of the circumstances in which this occurs is when parsing text
like the following:

    final b = (final g, final ) = 55;

In this example, there is a parse error due to the fact that the user
is trying to declare variables inside a pattern assignment. But one of
those variables doesn't have a name yet, so the parser has created a
synthetic token for it whose name is an empty string.

To avoid a crash, we must supply some name to the error message
template, so the name we supply is `(unnamed)`.

Fixes #54178.

Bug: #54178
Change-Id: Iabf3263ee1f56a89d1a69bcd74a17a296d470b9c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/339661
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

Author: stereotype441

pkg/_fe_analyzer_shared/lib/src/parser/parser_impl.dart

@@ -9771,8 +9771,8 @@ class Parser {
         if (!isBareIdentifier) {
           reportRecoverableError(
               token,
-              codes.templatePatternAssignmentDeclaresVariable
-                  .withArguments(variableName));
+              codes.templatePatternAssignmentDeclaresVariable.withArguments(
+                  variableName.isEmpty ? '(unnamed)' : variableName));
         }
         break;
     }

pkg/analyzer/test/generated/patterns_parser_test.dart

@@ -7524,6 +7524,21 @@ ForStatement
 ''');
   }
 
+  test_patternAssignment_declaresVariableWithMissingName() {
+    // Test case from https://github.com/dart-lang/sdk/issues/54178.
+    _parse('''
+void main() {
+  final b = (final g, final ) = 55;
+}
+''', errors: [
+      error(ParserErrorCode.PATTERN_ASSIGNMENT_DECLARES_VARIABLE, 33, 1),
+      error(ParserErrorCode.MISSING_IDENTIFIER, 42, 1),
+      error(ParserErrorCode.PATTERN_ASSIGNMENT_DECLARES_VARIABLE, 42, 1),
+    ]);
+    // No assertion on the parsed node text; all we are concerned with is that
+    // the parser doesn't crash.
+  }
+
   test_patternVariableDeclaration_inClass() {
     // If a pattern variable declaration appears outside a function or method,
     // the parser recovers by replacing the pattern with a synthetic identifier,