[linter] Dot shorthands: Update unnecessary_const lint.

Update `unnecessary_const` so it fires when we explicitly write a const dot shorthand in a constant context.

Unit tests added and passing.

Fixes: #60912
Bug: #60893
Change-Id: I5d2768dae4c55a3437fe2ec063af021d47721d98
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/434580
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Kallen Tu <[email protected]>

Author: kallentu

pkg/linter/lib/src/rules/unnecessary_const.dart

@@ -25,6 +25,7 @@ class UnnecessaryConst extends LintRule {
   @override
   void registerNodeProcessors(NodeLintRegistry registry, RuleContext context) {
     var visitor = _Visitor(this);
+    registry.addDotShorthandConstructorInvocation(this, visitor);
     registry.addInstanceCreationExpression(this, visitor);
     registry.addListLiteral(this, visitor);
     registry.addRecordLiteral(this, visitor);
@@ -36,6 +37,17 @@ class _Visitor extends SimpleAstVisitor<void> {
   final LintRule rule;
   _Visitor(this.rule);
 
+  @override
+  void visitDotShorthandConstructorInvocation(
+    DotShorthandConstructorInvocation node,
+  ) {
+    var constKeyword = node.constKeyword;
+    if (constKeyword == null) return;
+    if (node.inConstantContext) {
+      rule.reportAtToken(constKeyword);
+    }
+  }
+
   @override
   void visitInstanceCreationExpression(InstanceCreationExpression node) {
     var keyword = node.keyword;

pkg/linter/test/rules/unnecessary_const_test.dart

@@ -18,6 +18,32 @@ class UnnecessaryConstPatternsTest extends LintRuleTest {
   @override
   String get lintRule => LintNames.unnecessary_const;
 
+  test_case_constConstructor_dotShorthand_named_ok() async {
+    await assertNoDiagnostics(r'''
+class C {
+  const C.named();
+}
+f(C c) {
+  switch (c) {
+    case const .named():
+  }
+}
+''');
+  }
+
+  test_case_constConstructor_dotShorthand_ok() async {
+    await assertNoDiagnostics(r'''
+class C {
+  const C();
+}
+f(C c) {
+  switch (c) {
+    case const .new():
+  }
+}
+''');
+  }
+
   test_case_constConstructor_ok() async {
     await assertNoDiagnostics(r'''
 class C {
@@ -73,6 +99,30 @@ const c = const C();
     );
   }
 
+  test_constConstructor_dotShorthand() async {
+    await assertDiagnostics(
+      r'''
+class C {
+  const C();
+}
+const C c = const .new();
+''',
+      [lint(37, 5)],
+    );
+  }
+
+  test_constConstructor_dotShorthand_named() async {
+    await assertDiagnostics(
+      r'''
+class C {
+  const C.named();
+}
+const C c = const .named();
+''',
+      [lint(43, 5)],
+    );
+  }
+
   test_listLiteral() async {
     await assertDiagnostics(
       r'''