improve fasta export directive recovery

pkg/analyzer/lib/src/fasta/ast_builder.dart

Index: pkg/analyzer/lib/src/fasta/ast_builder.dart
diff --git a/pkg/analyzer/lib/src/fasta/ast_builder.dart b/pkg/analyzer/lib/src/fasta/ast_builder.dart
index 7580511ad0e7d1d44dadacb5129acc57205adbaa..80dd6fa415e4b7a3e3f1732b4148ac7759b06e9d 100644
--- a/pkg/analyzer/lib/src/fasta/ast_builder.dart
+++ b/pkg/analyzer/lib/src/fasta/ast_builder.dart
@@ -1138,6 +1138,7 @@ class AstBuilder extends ScopeListener {
 
   @override
   void beginCompilationUnit(Token token) {
+    super.beginCompilationUnit(token);
     push(token);
   }
 
@@ -1896,15 +1897,20 @@ class AstBuilder extends ScopeListener {
             ParserErrorCode.EXPECTED_STRING_LITERAL, charOffset, 1);
         return;
       case "UNEXPECTED_TOKEN":
-        var text = arguments['string'];
+        String text = arguments['string'];
         if (text == null) {
           Token token = arguments['token'];
           if (token != null) {
             text = token.lexeme;
           }
         }
-        errorReporter?.reportErrorForOffset(
-            ParserErrorCode.UNEXPECTED_TOKEN, charOffset, 1, [text]);
+        if (text == ';') {
+          errorReporter?.reportErrorForOffset(
+              ParserErrorCode.EXPECTED_TOKEN, charOffset, text.length, [text]);
+        } else {
+          errorReporter?.reportErrorForOffset(
+              ParserErrorCode.UNEXPECTED_TOKEN, charOffset, 1, [text]);
+        }
         return;
       default:
       // fall through