Implement search for direct subtypes and enable `search.getTypeHierarchy` request.

pkg/analyzer/test/src/dart/analysis/search_test.dart

Index: pkg/analyzer/test/src/dart/analysis/search_test.dart
diff --git a/pkg/analyzer/test/src/dart/analysis/search_test.dart b/pkg/analyzer/test/src/dart/analysis/search_test.dart
index 22434b927ec79ac3594e4c65bc3ab277830c176c..3b830fb9d240652dc1b6a0729df9973295c5be1b 100644
--- a/pkg/analyzer/test/src/dart/analysis/search_test.dart
+++ b/pkg/analyzer/test/src/dart/analysis/search_test.dart
@@ -886,6 +886,25 @@ foo<T>(T a) {
     await _verifyReferences(element, expected);
   }
 
+  test_searchSubtypes() async {
+    await _resolveTestUnit('''
+class T {}
+class A extends T {} // A
+class B = Object with T; // B
+class C implements T {} // C
+''');
+    ClassElement element = _findElement('T');
+    ClassElement a = _findElement('A');
+    ClassElement b = _findElement('B');
+    ClassElement c = _findElement('C');
+    var expected = [
+      _expectId(a, SearchResultKind.REFERENCE, 'T {} // A'),
+      _expectId(b, SearchResultKind.REFERENCE, 'T; // B'),
+      _expectId(c, SearchResultKind.REFERENCE, 'T {} // C'),
+    ];
+    await _verifyReferences(element, expected);
+  }
+
   ExpectedResult _expectId(
       Element enclosingElement, SearchResultKind kind, String search,
       {int length, bool isResolved: true, bool isQualified: false}) {