[wildcards] Support wildcard import prefixes

From the [spec](https://github.com/dart-lang/language/blob/main/working/wildcards/feature-specification.md#imports),

> Import prefixes named _ are non-binding but will provide access to the non-private extensions in that library.

Fixes: #56245

Change-Id: Ia38f0fa9f1f2daba8ad93ec844fa0b23ed77d2f8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/375762
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Phil Quitslund <[email protected]>

Author: pq

pkg/analyzer/lib/src/dart/element/extensions.dart

@@ -122,6 +122,7 @@ extension ElementExtension on Element {
   bool get isWildcardVariable =>
       name == '_' &&
       (this is LocalVariableElement ||
+          this is PrefixElement ||
           this is TypeParameterElement ||
           (this is ParameterElement &&
               this is! FieldFormalParameterElement &&

pkg/analyzer/lib/src/dart/element/scope.dart

@@ -128,7 +128,11 @@ class LibraryOrAugmentationScope extends EnclosedScope {
   ) : super(importScope) {
     extensions.addAll(importScope.extensions);
 
-    _container.prefixes.forEach(_addGetter);
+    for (var prefix in _container.prefixes) {
+      if (!prefix.isWildcardVariable) {
+        _addGetter(prefix);
+      }
+    }
     _container.library.units.forEach(_addUnitElements);
 
     // Add implicit 'dart:core' declarations.

pkg/analyzer/test/src/dart/resolution/library_import_prefix_test.dart

@@ -140,4 +140,75 @@ main() {
     assertElement(pRef, findElement.prefix('p'));
     assertTypeNull(pRef);
   }
+
+  test_wildcardResolution() async {
+    newFile('$testPackageLibPath/a.dart', r'''
+extension ExtendedString on String {
+  bool get stringExt => true;
+}
+
+var a = 0;
+''');
+
+    newFile('$testPackageLibPath/b.dart', r'''
+extension ExtendedString2 on String {
+  bool get stringExt2 => true;
+}
+''');
+
+    // Import prefixes named `_` provide access to non-private extensions
+    // in the imported library but are non-binding.
+    await assertErrorsInCode(r'''
+import 'a.dart' as _;
+import 'b.dart' as _;
+
+f() {
+  ''.stringExt;
+  ''.stringExt2;
+  _.a;
+}
+''', [
+      // String extensions are found but `_` is not bound.
+      error(CompileTimeErrorCode.UNDEFINED_IDENTIFIER, 86, 1),
+    ]);
+  }
+
+  test_wildcardResolution_preWildcards() async {
+    newFile('$testPackageLibPath/a.dart', r'''
+extension ExtendedString on String {
+  bool get stringExt => true;
+}
+
+var a = 0;
+''');
+
+    newFile('$testPackageLibPath/b.dart', r'''
+extension ExtendedString on String {
+  bool get stringExt2 => true;
+}
+''');
+
+    await assertNoErrorsInCode(r'''
+// @dart = 3.4
+// (pre wildcard-variables)
+
+import 'a.dart' as _;
+import 'b.dart' as _;
+
+f() {
+  ''.stringExt;
+  ''.stringExt2;
+  _.a;
+}
+''');
+
+    // `_` is bound so `a` resolves to the int declared in `a.dart`.
+    var node = findNode.simple('a;');
+    assertResolvedNodeText(node, r'''
+SimpleIdentifier
+  token: a
+  staticElement: package:test/a.dart::@getter::a
+  staticType: int
+''');
+  }
 }