Handle imports with "as" and "if" clauses in either order. (#1555)

PR #1550 fixed the bug where if you had both clauses, it would output
them in the *wrong* order. But it turns out that the parser today allows
them in *either* order even though the language spec doesn't.

Since I've seen a handful of cases like this in the wild, instead of
just failing to format, simply preserve the clause order that the user
wrote.

Author: munificent

lib/src/front_end/piece_factory.dart

@@ -812,8 +812,26 @@ mixin PieceFactory {
         pieces.visit(directive.uri);
       });
 
-      // Include the `as` clause.
+      // Add all of the clauses and combinators.
       var clauses = <Piece>[];
+
+      // The language specifies that configurations must appear after any `as`
+      // clause but the parser incorrectly accepts them before it and code in
+      // the wild relies on that. Instead of failing with an "unexpected output"
+      // error, just preserve the order of the clauses if they are out of order.
+      // See: https://github.com/dart-lang/sdk/issues/56641
+      var wroteConfigurations = false;
+      if (directive.configurations.isNotEmpty &&
+          asKeyword != null &&
+          directive.configurations.first.ifKeyword.offset < asKeyword.offset) {
+        for (var configuration in directive.configurations) {
+          clauses.add(nodePiece(configuration));
+        }
+
+        wroteConfigurations = true;
+      }
+
+      // Include the `as` clause.
       if (asKeyword != null) {
         clauses.add(pieces.build(() {
           pieces.token(deferredKeyword, spaceAfter: true);
@@ -824,8 +842,10 @@ mixin PieceFactory {
       }
 
       // Include any `if` clauses.
-      for (var configuration in directive.configurations) {
-        clauses.add(nodePiece(configuration));
+      if (!wroteConfigurations) {
+        for (var configuration in directive.configurations) {
+          clauses.add(nodePiece(configuration));
+        }
       }
 
       // Include the `show` and `hide` clauses.

lib/src/short/source_visitor.dart

@@ -1955,6 +1955,19 @@ class SourceVisitor extends ThrowingAstVisitor {
       space();
       visit(node.uri);
 
+      // The language specifies that configurations must appear after any `as`
+      // clause but the parser incorrectly accepts them before it and code in
+      // the wild relies on that. Instead of failing with an "unexpected output"
+      // error, just preserve the order of the clauses if they are out of order.
+      // See: https://github.com/dart-lang/sdk/issues/56641
+      var wroteConfigurations = false;
+      if (node.asKeyword case var asKeyword?
+          when node.configurations.isNotEmpty &&
+              node.configurations.first.ifKeyword.offset < asKeyword.offset) {
+        _visitConfigurations(node.configurations);
+        wroteConfigurations = true;
+      }
+
       if (node.asKeyword != null) {
         soloSplit();
         token(node.deferredKeyword, after: space);
@@ -1963,7 +1976,10 @@ class SourceVisitor extends ThrowingAstVisitor {
         visit(node.prefix);
       }
 
-      _visitConfigurations(node.configurations);
+      if (!wroteConfigurations) {
+        _visitConfigurations(node.configurations);
+      }
+
       _visitCombinators(node.combinators);
     });
   }

test/short/whitespace/directives.unit

@@ -70,4 +70,13 @@ part of 'uri.dart';
 import 'foo.dart' as foo if (config == 'value') 'other.dart';
 <<<
 import 'foo.dart' as foo
-    if (config == 'value') 'other.dart';
+    if (config == 'value') 'other.dart';
+>>> Configuration before prefix.
+### This violates the language spec, but the parser currently allows it without
+### reporting an error and code in the wild relies on that. So the formatter
+### handles it gracefully. See: https://github.com/dart-lang/sdk/issues/56641
+import 'foo.dart' if (config == 'value') 'other.dart' as foo;
+<<<
+import 'foo.dart'
+    if (config == 'value') 'other.dart'
+    as foo;

test/tall/top_level/import.unit

@@ -73,4 +73,13 @@ import 'foo.dart' as foo if (config == 'value') 'other.dart';
 <<<
 import 'foo.dart'
     as foo
-    if (config == 'value') 'other.dart';
+    if (config == 'value') 'other.dart';
+>>> Configuration before prefix.
+### This violates the language spec, but the parser currently allows it without
+### reporting an error and code in the wild relies on that. So the formatter
+### handles it gracefully. See: https://github.com/dart-lang/sdk/issues/56641
+import 'foo.dart' if (config == 'value') 'other.dart' as foo;
+<<<
+import 'foo.dart'
+    if (config == 'value') 'other.dart'
+    as foo;