[analyzer] Dot shorthands: Allow index expressions.

`IndexExpressionImpl`s now have the `DotShorthandMixin` applied. We save the context at the point of resolving an index expression to be used for resolving a dot shorthand head later.

Unit tests added. There's multiple co19 tests that will start passing, but rely on https://dart-review.googlesource.com/c/sdk/+/425181.

Bug: https://github.com/dart-lang/sdk/issues/59835
Change-Id: I08076bb437bad1955d353a24fc119466b7dfad61
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/425360
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Kallen Tu <kallentu@google.com>

pkg/analyzer/lib/src/dart/ast/ast.dart

@@ -10091,7 +10091,7 @@ abstract final class IndexExpression
 }
 
 final class IndexExpressionImpl extends ExpressionImpl
-    with NullShortableExpressionImpl
+    with NullShortableExpressionImpl, DotShorthandMixin
     implements IndexExpression {
   @override
   Token? period;

pkg/analyzer/lib/src/generated/resolver.dart

@@ -3074,6 +3074,12 @@ class ResolverVisitor extends ThrowingAstVisitor<void>
     TypeImpl contextType = UnknownInferredType.instance,
   }) {
     inferenceLogWriter?.enterExpression(node, contextType);
+
+    // If [isDotShorthand] is set, cache the context type for resolution.
+    if (isDotShorthand(node)) {
+      pushDotShorthandContext(node, SharedTypeSchemaView(contextType));
+    }
+
     checkUnreachableNode(node);
 
     var target = node.target;
@@ -3132,6 +3138,11 @@ class ResolverVisitor extends ThrowingAstVisitor<void>
     nullShortingTermination(node, rewrittenExpression: replacement);
     _insertImplicitCallReference(replacement, contextType: contextType);
     nullSafetyDeadCodeVerifier.verifyIndexExpression(node);
+
+    if (isDotShorthand(node)) {
+      popDotShorthandContext();
+    }
+
     inferenceLogWriter?.exitExpression(node);
   }
 

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

@@ -321,6 +321,35 @@ DotShorthandInvocation
 ''');
   }
 
+  test_equality_indexExpression() async {
+    await assertNoErrorsInCode(r'''
+class C {
+  int x;
+  C(this.x);
+  static List<C> instances() => [C(1)];
+}
+
+void main() {
+  print(C(1) == .instances()[0]);
+}
+''');
+
+    var identifier = findNode.singleDotShorthandInvocation;
+    assertResolvedNodeText(identifier, r'''
+DotShorthandInvocation
+  period: .
+  memberName: SimpleIdentifier
+    token: instances
+    element: <testLibraryFragment>::@class::C::@method::instances#element
+    staticType: List<C> Function()
+  argumentList: ArgumentList
+    leftParenthesis: (
+    rightParenthesis: )
+  staticInvokeType: List<C> Function()
+  staticType: List<C>
+''');
+  }
+
   test_extensionType() async {
     await assertNoErrorsInCode(r'''
 extension type C(int integer) {

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

@@ -270,6 +270,31 @@ DotShorthandPropertyAccess
 ''');
   }
 
+  test_equality_indexExpression() async {
+    await assertNoErrorsInCode(r'''
+class C {
+  int x;
+  C(this.x);
+  static List<C> instances = [C(1)];
+}
+
+void main() {
+  print(C(1) == .instances[0]);
+}
+''');
+
+    var identifier = findNode.singleDotShorthandPropertyAccess;
+    assertResolvedNodeText(identifier, r'''
+DotShorthandPropertyAccess
+  period: .
+  propertyName: SimpleIdentifier
+    token: instances
+    element: <testLibraryFragment>::@class::C::@getter::instances#element
+    staticType: List<C>
+  staticType: List<C>
+''');
+  }
+
   test_equality_pattern() async {
     await assertNoErrorsInCode('''
 enum Color { red, blue }