Update ImportedElementsComputer for primary constructors

This CL is to resolve a TODO. I couldn't find a way to trigger any bugs
in the code, but I did add a test. I believe that the reason I couldn't
trigger a bug is because any constructor that has a null `typeName`
will not have a simple identifier in a place that would allow
`_isConstructorDeclarationReturnType` to be invoked.

I realized, while looking at the code, that the type check is
unnecessary. If the type name is null, then the test will somply return
`false`, which is what we'd expect it to do. Given that it isn't needed,
I decided to remove it.

Change-Id: Ib36aaf16475db0db5817e65a8b8bfbb8aa834dbd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/505501
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

pkg/analysis_server/lib/src/computer/imported_elements_computer.dart

@@ -148,7 +148,6 @@ class _Visitor extends UnifyingAstVisitor<void> {
 
   static bool _isConstructorDeclarationReturnType(SimpleIdentifier node) {
     var parent = node.parent;
-    // TODO(scheglov): support primary constructors
-    return parent is ConstructorDeclaration && parent.typeName! == node;
+    return parent is ConstructorDeclaration && parent.typeName == node;
   }
 }

pkg/analysis_server/test/src/computer/imported_elements_computer_test.dart

@@ -176,6 +176,19 @@ blankLine() {
     expect(importedElements, hasLength(0));
   }
 
+  Future<void> test_none_constructorDeclaration_withNew() async {
+    var content = '''
+[!
+class A {
+  new ();
+  new named();
+}
+!]
+''';
+    await _computeElements(content);
+    expect(importedElements, hasLength(0));
+  }
+
   Future<void> test_none_constructorDeclarationReturnType() async {
     var content = '''
 [!