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

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

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/dart/analysis/driver.dart';
 import 'package:analyzer/src/dart/analysis/search.dart';
@@ skipping 868 matching lines @@
     TypeParameterElement element = _findElement('T');
     Element a = _findElement('a');
     Element b = _findElement('b');
     var expected = [
       _expectId(a, SearchResultKind.REFERENCE, 'T a'),
       _expectId(b, SearchResultKind.REFERENCE, 'T b'),
     ];
     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}) {
     int offset = findOffset(search);
     if (length == null) {
       length = getLeadingIdentifierLength(search);
     }
     return new ExpectedResult(enclosingElement, kind, offset, length,
         isResolved: isResolved, isQualified: isQualified);
   }
@@ skipping 34 matching lines @@
     List<SearchResult> results = await driver.search.references(element);
     _assertResults(results, expectedMatches);
     expect(results, hasLength(expectedMatches.length));
   }
 
   static void _assertResults(
       List<SearchResult> matches, List<ExpectedResult> expectedMatches) {
     expect(matches, unorderedEquals(expectedMatches));
   }
 }