diff --git a/pkg/analyzer/lib/src/dart/analysis/driver.dart b/pkg/analyzer/lib/src/dart/analysis/driver.dart
index fc72f2b..3269d69 100644
--- a/pkg/analyzer/lib/src/dart/analysis/driver.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/driver.dart
@@ -93,7 +93,7 @@
   /**
    * The version of data format, should be incremented on every format change.
    */
-  static const int DATA_VERSION = 69;
+  static const int DATA_VERSION = 70;
 
   /**
    * The number of exception contexts allowed to write. Once this field is
diff --git a/pkg/analyzer/lib/src/dart/analysis/index.dart b/pkg/analyzer/lib/src/dart/analysis/index.dart
index 69dd439..4e620344 100644
--- a/pkg/analyzer/lib/src/dart/analysis/index.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/index.dart
@@ -212,7 +212,7 @@
   /**
    * All subtypes declared in the unit.
    */
-  final List<AnalysisDriverSubtypeBuilder> subtypes = [];
+  final List<_SubtypeInfo> subtypes = [];
 
   /**
    * The [_StringInfo] to use for `null` strings.
@@ -236,12 +236,25 @@
     nameRelations.add(new _NameRelationInfo(nameId, kind, offset, isQualified));
   }
 
+  void addSubtype(String name, List<String> members, List<String> supertypes) {
+    for (var supertype in supertypes) {
+      subtypes.add(
+        new _SubtypeInfo(
+          _getStringInfo(supertype),
+          _getStringInfo(name),
+          members.map(_getStringInfo).toList(),
+        ),
+      );
+    }
+  }
+
   /**
    * Index the [unit] and assemble a new [AnalysisDriverUnitIndexBuilder].
    */
   AnalysisDriverUnitIndexBuilder assemble(CompilationUnit unit) {
     unit.accept(new _IndexContributor(this));
-    // sort strings end set IDs
+
+    // Sort strings and set IDs.
     List<_StringInfo> stringInfoList = stringMap.values.toList();
     stringInfoList.sort((a, b) {
       return a.value.compareTo(b.value);
@@ -249,16 +262,17 @@
     for (int i = 0; i < stringInfoList.length; i++) {
       stringInfoList[i].id = i;
     }
-    // sort elements and set IDs
+
+    // Sort elements and set IDs.
     List<_ElementInfo> elementInfoList = elementMap.values.toList();
     elementInfoList.sort((a, b) {
       int delta;
       delta = a.nameIdUnitMember.id - b.nameIdUnitMember.id;
-      if (delta != null) {
+      if (delta != 0) {
         return delta;
       }
       delta = a.nameIdClassMember.id - b.nameIdClassMember.id;
-      if (delta != null) {
+      if (delta != 0) {
         return delta;
       }
       return a.nameIdParameter.id - b.nameIdParameter.id;
@@ -266,6 +280,7 @@
     for (int i = 0; i < elementInfoList.length; i++) {
       elementInfoList[i].id = i;
     }
+
     // Sort element and name relations.
     elementRelations.sort((a, b) {
       return a.elementInfo.id - b.elementInfo.id;
@@ -273,6 +288,12 @@
     nameRelations.sort((a, b) {
       return a.nameInfo.id - b.nameInfo.id;
     });
+
+    // Sort subtypes by supertypes.
+    subtypes.sort((a, b) {
+      return a.supertype.id - b.supertype.id;
+    });
+
     return new AnalysisDriverUnitIndexBuilder(
         strings: stringInfoList.map((s) => s.value).toList(),
         nullStringId: nullString.id,
@@ -297,7 +318,13 @@
         usedNameOffsets: nameRelations.map((r) => r.offset).toList(),
         usedNameIsQualifiedFlags:
             nameRelations.map((r) => r.isQualified).toList(),
-        subtypes: subtypes);
+        supertypes: subtypes.map((subtype) => subtype.supertype.id).toList(),
+        subtypes: subtypes.map((subtype) {
+          return new AnalysisDriverSubtypeBuilder(
+            name: subtype.name.id,
+            members: subtype.members.map((member) => member.id).toList(),
+          );
+        }).toList());
   }
 
   /**
@@ -806,8 +833,7 @@
     supertypes.sort();
     members.sort();
 
-    assembler.subtypes.add(new AnalysisDriverSubtypeBuilder(
-        name: name, supertypes: supertypes, members: members));
+    assembler.addSubtype(name, members, supertypes);
   }
 
   /**
@@ -938,3 +964,25 @@
 
   _StringInfo(this.value);
 }
+
+/**
+ * Information about a subtype in the index.
+ */
+class _SubtypeInfo {
+  /**
+   * The identifier of a direct supertype.
+   */
+  final _StringInfo supertype;
+
+  /**
+   * The name of the class.
+   */
+  final _StringInfo name;
+
+  /**
+   * The names of defined instance members.
+   */
+  final List<_StringInfo> members;
+
+  _SubtypeInfo(this.supertype, this.name, this.members);
+}
diff --git a/pkg/analyzer/lib/src/dart/analysis/search.dart b/pkg/analyzer/lib/src/dart/analysis/search.dart
index 447d1f7..9d1ff41 100644
--- a/pkg/analyzer/lib/src/dart/analysis/search.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/search.dart
@@ -378,8 +378,6 @@
    */
   Future<List<SubtypeResult>> subtypes(
       {ClassElement type, SubtypeResult subtype}) async {
-    // TODO(brianwilkerson) Determine whether this await is necessary.
-    await null;
     String name;
     String id;
     if (type != null) {
@@ -401,17 +399,8 @@
       for (FileState file in files) {
         AnalysisDriverUnitIndex index = await _driver.getIndex(file.path);
         if (index != null) {
-          for (AnalysisDriverSubtype subtype in index.subtypes) {
-            if (subtype.supertypes.contains(id)) {
-              FileState library = file.library ?? file;
-              results.add(new SubtypeResult(
-                library.uriStr,
-                '${library.uriStr};${file.uriStr};${subtype.name}',
-                subtype.name,
-                subtype.members,
-              ));
-            }
-          }
+          var request = new _IndexRequest(index);
+          request.addSubtypes(id, results, file);
         }
       }
     }
@@ -987,6 +976,32 @@
 
   _IndexRequest(this.index);
 
+  void addSubtypes(
+      String superIdString, List<SubtypeResult> results, FileState file) {
+    var superId = getStringId(superIdString);
+    if (superId == -1) {
+      return;
+    }
+
+    var superIndex = _findFirstOccurrence(index.supertypes, superId);
+    if (superIndex == -1) {
+      return;
+    }
+
+    var library = file.library ?? file;
+    for (; index.supertypes[superIndex] == superId; superIndex++) {
+      var subtype = index.subtypes[superIndex];
+      var name = index.strings[subtype.name];
+      var subId = '${library.uriStr};${file.uriStr};$name';
+      results.add(new SubtypeResult(
+        library.uriStr,
+        subId,
+        name,
+        subtype.members.map((m) => index.strings[m]).toList(),
+      ));
+    }
+  }
+
   /**
    * Return the [element]'s identifier in the [index] or `-1` if the
    * [element] is not referenced in the [index].
diff --git a/pkg/analyzer/lib/src/summary/format.dart b/pkg/analyzer/lib/src/summary/format.dart
index 610ae0d..cda912f 100644
--- a/pkg/analyzer/lib/src/summary/format.dart
+++ b/pkg/analyzer/lib/src/summary/format.dart
@@ -672,47 +672,37 @@
 class AnalysisDriverSubtypeBuilder extends Object
     with _AnalysisDriverSubtypeMixin
     implements idl.AnalysisDriverSubtype {
-  List<String> _members;
-  String _name;
-  List<String> _supertypes;
+  List<int> _members;
+  int _name;
 
   @override
-  List<String> get members => _members ??= <String>[];
+  List<int> get members => _members ??= <int>[];
 
   /**
    * The names of defined instance members.
+   * They are indexes into [AnalysisDriverUnitError.strings] list.
    * The list is sorted in ascending order.
    */
-  void set members(List<String> value) {
+  void set members(List<int> value) {
+    assert(value == null || value.every((e) => e >= 0));
     this._members = value;
   }
 
   @override
-  String get name => _name ??= '';
+  int get name => _name ??= 0;
 
   /**
    * The name of the class.
+   * It is an index into [AnalysisDriverUnitError.strings] list.
    */
-  void set name(String value) {
+  void set name(int value) {
+    assert(value == null || value >= 0);
     this._name = value;
   }
 
-  @override
-  List<String> get supertypes => _supertypes ??= <String>[];
-
-  /**
-   * The identifiers of the direct supertypes.
-   * The list is sorted in ascending order.
-   */
-  void set supertypes(List<String> value) {
-    this._supertypes = value;
-  }
-
-  AnalysisDriverSubtypeBuilder(
-      {List<String> members, String name, List<String> supertypes})
+  AnalysisDriverSubtypeBuilder({List<int> members, int name})
       : _members = members,
-        _name = name,
-        _supertypes = supertypes;
+        _name = name;
 
   /**
    * Flush [informative] data recursively.
@@ -723,49 +713,28 @@
    * Accumulate non-[informative] data into [signature].
    */
   void collectApiSignature(api_sig.ApiSignature signature) {
-    signature.addString(this._name ?? '');
-    if (this._supertypes == null) {
-      signature.addInt(0);
-    } else {
-      signature.addInt(this._supertypes.length);
-      for (var x in this._supertypes) {
-        signature.addString(x);
-      }
-    }
+    signature.addInt(this._name ?? 0);
     if (this._members == null) {
       signature.addInt(0);
     } else {
       signature.addInt(this._members.length);
       for (var x in this._members) {
-        signature.addString(x);
+        signature.addInt(x);
       }
     }
   }
 
   fb.Offset finish(fb.Builder fbBuilder) {
     fb.Offset offset_members;
-    fb.Offset offset_name;
-    fb.Offset offset_supertypes;
     if (!(_members == null || _members.isEmpty)) {
-      offset_members = fbBuilder
-          .writeList(_members.map((b) => fbBuilder.writeString(b)).toList());
-    }
-    if (_name != null) {
-      offset_name = fbBuilder.writeString(_name);
-    }
-    if (!(_supertypes == null || _supertypes.isEmpty)) {
-      offset_supertypes = fbBuilder
-          .writeList(_supertypes.map((b) => fbBuilder.writeString(b)).toList());
+      offset_members = fbBuilder.writeListUint32(_members);
     }
     fbBuilder.startTable();
     if (offset_members != null) {
-      fbBuilder.addOffset(2, offset_members);
+      fbBuilder.addOffset(1, offset_members);
     }
-    if (offset_name != null) {
-      fbBuilder.addOffset(0, offset_name);
-    }
-    if (offset_supertypes != null) {
-      fbBuilder.addOffset(1, offset_supertypes);
+    if (_name != null && _name != 0) {
+      fbBuilder.addUint32(0, _name);
     }
     return fbBuilder.endTable();
   }
@@ -788,29 +757,21 @@
 
   _AnalysisDriverSubtypeImpl(this._bc, this._bcOffset);
 
-  List<String> _members;
-  String _name;
-  List<String> _supertypes;
+  List<int> _members;
+  int _name;
 
   @override
-  List<String> get members {
-    _members ??= const fb.ListReader<String>(const fb.StringReader())
-        .vTableGet(_bc, _bcOffset, 2, const <String>[]);
+  List<int> get members {
+    _members ??=
+        const fb.Uint32ListReader().vTableGet(_bc, _bcOffset, 1, const <int>[]);
     return _members;
   }
 
   @override
-  String get name {
-    _name ??= const fb.StringReader().vTableGet(_bc, _bcOffset, 0, '');
+  int get name {
+    _name ??= const fb.Uint32Reader().vTableGet(_bc, _bcOffset, 0, 0);
     return _name;
   }
-
-  @override
-  List<String> get supertypes {
-    _supertypes ??= const fb.ListReader<String>(const fb.StringReader())
-        .vTableGet(_bc, _bcOffset, 1, const <String>[]);
-    return _supertypes;
-  }
 }
 
 abstract class _AnalysisDriverSubtypeMixin
@@ -819,8 +780,7 @@
   Map<String, Object> toJson() {
     Map<String, Object> _result = <String, Object>{};
     if (members.isNotEmpty) _result["members"] = members;
-    if (name != '') _result["name"] = name;
-    if (supertypes.isNotEmpty) _result["supertypes"] = supertypes;
+    if (name != 0) _result["name"] = name;
     return _result;
   }
 
@@ -828,7 +788,6 @@
   Map<String, Object> toMap() => {
         "members": members,
         "name": name,
-        "supertypes": supertypes,
       };
 
   @override
@@ -1048,6 +1007,7 @@
   int _nullStringId;
   List<String> _strings;
   List<AnalysisDriverSubtypeBuilder> _subtypes;
+  List<int> _supertypes;
   List<int> _unitLibraryUris;
   List<int> _unitUnitUris;
   List<bool> _usedElementIsQualifiedFlags;
@@ -1164,6 +1124,20 @@
   }
 
   @override
+  List<int> get supertypes => _supertypes ??= <int>[];
+
+  /**
+   * The identifiers of supertypes of elements at corresponding indexes
+   * in [subtypes].  They are indexes into [strings] list. The list is sorted
+   * in ascending order.  There might be more than one element with the same
+   * value if there is more than one subtype of this supertype.
+   */
+  void set supertypes(List<int> value) {
+    assert(value == null || value.every((e) => e >= 0));
+    this._supertypes = value;
+  }
+
+  @override
   List<int> get unitLibraryUris => _unitLibraryUris ??= <int>[];
 
   /**
@@ -1304,6 +1278,7 @@
       int nullStringId,
       List<String> strings,
       List<AnalysisDriverSubtypeBuilder> subtypes,
+      List<int> supertypes,
       List<int> unitLibraryUris,
       List<int> unitUnitUris,
       List<bool> usedElementIsQualifiedFlags,
@@ -1323,6 +1298,7 @@
         _nullStringId = nullStringId,
         _strings = strings,
         _subtypes = subtypes,
+        _supertypes = supertypes,
         _unitLibraryUris = unitLibraryUris,
         _unitUnitUris = unitUnitUris,
         _usedElementIsQualifiedFlags = usedElementIsQualifiedFlags,
@@ -1483,6 +1459,14 @@
         signature.addBool(x);
       }
     }
+    if (this._supertypes == null) {
+      signature.addInt(0);
+    } else {
+      signature.addInt(this._supertypes.length);
+      for (var x in this._supertypes) {
+        signature.addInt(x);
+      }
+    }
     if (this._subtypes == null) {
       signature.addInt(0);
     } else {
@@ -1506,6 +1490,7 @@
     fb.Offset offset_elementUnits;
     fb.Offset offset_strings;
     fb.Offset offset_subtypes;
+    fb.Offset offset_supertypes;
     fb.Offset offset_unitLibraryUris;
     fb.Offset offset_unitUnitUris;
     fb.Offset offset_usedElementIsQualifiedFlags;
@@ -1547,6 +1532,9 @@
       offset_subtypes = fbBuilder
           .writeList(_subtypes.map((b) => b.finish(fbBuilder)).toList());
     }
+    if (!(_supertypes == null || _supertypes.isEmpty)) {
+      offset_supertypes = fbBuilder.writeListUint32(_supertypes);
+    }
     if (!(_unitLibraryUris == null || _unitLibraryUris.isEmpty)) {
       offset_unitLibraryUris = fbBuilder.writeListUint32(_unitLibraryUris);
     }
@@ -1611,7 +1599,10 @@
       fbBuilder.addOffset(0, offset_strings);
     }
     if (offset_subtypes != null) {
-      fbBuilder.addOffset(18, offset_subtypes);
+      fbBuilder.addOffset(19, offset_subtypes);
+    }
+    if (offset_supertypes != null) {
+      fbBuilder.addOffset(18, offset_supertypes);
     }
     if (offset_unitLibraryUris != null) {
       fbBuilder.addOffset(2, offset_unitLibraryUris);
@@ -1680,6 +1671,7 @@
   int _nullStringId;
   List<String> _strings;
   List<idl.AnalysisDriverSubtype> _subtypes;
+  List<int> _supertypes;
   List<int> _unitLibraryUris;
   List<int> _unitUnitUris;
   List<bool> _usedElementIsQualifiedFlags;
@@ -1745,11 +1737,18 @@
   List<idl.AnalysisDriverSubtype> get subtypes {
     _subtypes ??= const fb.ListReader<idl.AnalysisDriverSubtype>(
             const _AnalysisDriverSubtypeReader())
-        .vTableGet(_bc, _bcOffset, 18, const <idl.AnalysisDriverSubtype>[]);
+        .vTableGet(_bc, _bcOffset, 19, const <idl.AnalysisDriverSubtype>[]);
     return _subtypes;
   }
 
   @override
+  List<int> get supertypes {
+    _supertypes ??= const fb.Uint32ListReader()
+        .vTableGet(_bc, _bcOffset, 18, const <int>[]);
+    return _supertypes;
+  }
+
+  @override
   List<int> get unitLibraryUris {
     _unitLibraryUris ??=
         const fb.Uint32ListReader().vTableGet(_bc, _bcOffset, 2, const <int>[]);
@@ -1849,6 +1848,7 @@
     if (strings.isNotEmpty) _result["strings"] = strings;
     if (subtypes.isNotEmpty)
       _result["subtypes"] = subtypes.map((_value) => _value.toJson()).toList();
+    if (supertypes.isNotEmpty) _result["supertypes"] = supertypes;
     if (unitLibraryUris.isNotEmpty)
       _result["unitLibraryUris"] = unitLibraryUris;
     if (unitUnitUris.isNotEmpty) _result["unitUnitUris"] = unitUnitUris;
@@ -1885,6 +1885,7 @@
         "nullStringId": nullStringId,
         "strings": strings,
         "subtypes": subtypes,
+        "supertypes": supertypes,
         "unitLibraryUris": unitLibraryUris,
         "unitUnitUris": unitUnitUris,
         "usedElementIsQualifiedFlags": usedElementIsQualifiedFlags,
diff --git a/pkg/analyzer/lib/src/summary/format.fbs b/pkg/analyzer/lib/src/summary/format.fbs
index a37da95..0f70322 100644
--- a/pkg/analyzer/lib/src/summary/format.fbs
+++ b/pkg/analyzer/lib/src/summary/format.fbs
@@ -987,20 +987,16 @@
 table AnalysisDriverSubtype {
   /**
    * The names of defined instance members.
+   * They are indexes into [AnalysisDriverUnitError.strings] list.
    * The list is sorted in ascending order.
    */
-  members:[string] (id: 2);
+  members:[uint] (id: 1);
 
   /**
    * The name of the class.
+   * It is an index into [AnalysisDriverUnitError.strings] list.
    */
-  name:string (id: 0);
-
-  /**
-   * The identifiers of the direct supertypes.
-   * The list is sorted in ascending order.
-   */
-  supertypes:[string] (id: 1);
+  name:uint (id: 0);
 }
 
 /**
@@ -1089,7 +1085,15 @@
   /**
    * The list of classes declared in the unit.
    */
-  subtypes:[AnalysisDriverSubtype] (id: 18);
+  subtypes:[AnalysisDriverSubtype] (id: 19);
+
+  /**
+   * The identifiers of supertypes of elements at corresponding indexes
+   * in [subtypes].  They are indexes into [strings] list. The list is sorted
+   * in ascending order.  There might be more than one element with the same
+   * value if there is more than one subtype of this supertype.
+   */
+  supertypes:[uint] (id: 18);
 
   /**
    * Each item of this list corresponds to the library URI of a unique library
diff --git a/pkg/analyzer/lib/src/summary/idl.dart b/pkg/analyzer/lib/src/summary/idl.dart
index c9ca5fd..4a9640c 100644
--- a/pkg/analyzer/lib/src/summary/idl.dart
+++ b/pkg/analyzer/lib/src/summary/idl.dart
@@ -134,23 +134,18 @@
 abstract class AnalysisDriverSubtype extends base.SummaryClass {
   /**
    * The names of defined instance members.
-   * The list is sorted in ascending order.
-   */
-  @Id(2)
-  List<String> get members;
-
-  /**
-   * The name of the class.
-   */
-  @Id(0)
-  String get name;
-
-  /**
-   * The identifiers of the direct supertypes.
+   * They are indexes into [AnalysisDriverUnitError.strings] list.
    * The list is sorted in ascending order.
    */
   @Id(1)
-  List<String> get supertypes;
+  List<int> get members;
+
+  /**
+   * The name of the class.
+   * It is an index into [AnalysisDriverUnitError.strings] list.
+   */
+  @Id(0)
+  int get name;
 }
 
 /**
@@ -255,10 +250,19 @@
   /**
    * The list of classes declared in the unit.
    */
-  @Id(18)
+  @Id(19)
   List<AnalysisDriverSubtype> get subtypes;
 
   /**
+   * The identifiers of supertypes of elements at corresponding indexes
+   * in [subtypes].  They are indexes into [strings] list. The list is sorted
+   * in ascending order.  There might be more than one element with the same
+   * value if there is more than one subtype of this supertype.
+   */
+  @Id(18)
+  List<int> get supertypes;
+
+  /**
    * Each item of this list corresponds to the library URI of a unique library
    * specific unit referenced in the index.  It is an index into [strings] list.
    */
diff --git a/pkg/analyzer/test/src/dart/analysis/index_test.dart b/pkg/analyzer/test/src/dart/analysis/index_test.dart
index 179e4c4..e3106d1 100644
--- a/pkg/analyzer/test/src/dart/analysis/index_test.dart
+++ b/pkg/analyzer/test/src/dart/analysis/index_test.dart
@@ -1014,27 +1014,20 @@
 }
 ''');
 
-    {
-      AnalysisDriverSubtype X =
-          index.subtypes.singleWhere((t) => t.name == 'X');
-      expect(X.supertypes, ['$libP;A']);
-      expect(X.members, ['field1', 'field2', 'getter1', 'method1', 'setter1']);
-    }
+    expect(index.supertypes, hasLength(6));
+    expect(index.subtypes, hasLength(6));
 
-    {
-      AnalysisDriverSubtype Y =
-          index.subtypes.singleWhere((t) => t.name == 'Y');
-      expect(
-          Y.supertypes, ['dart:core;dart:core;Object', '$libP;B', '$libP;C']);
-      expect(Y.members, ['methodY']);
-    }
-
-    {
-      AnalysisDriverSubtype Z =
-          index.subtypes.singleWhere((t) => t.name == 'Z');
-      expect(Z.supertypes, ['$libP;D', '$libP;E']);
-      expect(Z.members, ['methodZ']);
-    }
+    _assertSubtype(0, 'dart:core;dart:core;Object', 'Y', ['methodY']);
+    _assertSubtype(
+      1,
+      '$libP;A',
+      'X',
+      ['field1', 'field2', 'getter1', 'method1', 'setter1'],
+    );
+    _assertSubtype(2, '$libP;B', 'Y', ['methodY']);
+    _assertSubtype(3, '$libP;C', 'Y', ['methodY']);
+    _assertSubtype(4, '$libP;D', 'Z', ['methodZ']);
+    _assertSubtype(5, '$libP;E', 'Z', ['methodZ']);
   }
 
   test_subtypes_classTypeAlias() async {
@@ -1052,19 +1045,16 @@
 class Y = A with B implements C, D;
 ''');
 
-    {
-      AnalysisDriverSubtype X =
-          index.subtypes.singleWhere((t) => t.name == 'X');
-      expect(X.supertypes, ['$libP;A', '$libP;B', '$libP;C']);
-      expect(X.members, isEmpty);
-    }
+    expect(index.supertypes, hasLength(7));
+    expect(index.subtypes, hasLength(7));
 
-    {
-      AnalysisDriverSubtype Y =
-          index.subtypes.singleWhere((t) => t.name == 'Y');
-      expect(Y.supertypes, ['$libP;A', '$libP;B', '$libP;C', '$libP;D']);
-      expect(Y.members, isEmpty);
-    }
+    _assertSubtype(0, '$libP;A', 'X', []);
+    _assertSubtype(1, '$libP;A', 'Y', []);
+    _assertSubtype(2, '$libP;B', 'X', []);
+    _assertSubtype(3, '$libP;B', 'Y', []);
+    _assertSubtype(4, '$libP;C', 'X', []);
+    _assertSubtype(5, '$libP;C', 'Y', []);
+    _assertSubtype(6, '$libP;D', 'Y', []);
   }
 
   test_subtypes_dynamic() async {
@@ -1074,9 +1064,8 @@
 }
 ''');
 
-    AnalysisDriverSubtype X = index.subtypes.singleWhere((t) => t.name == 'X');
-    expect(X.supertypes, isEmpty);
-    expect(X.members, ['foo']);
+    expect(index.supertypes, isEmpty);
+    expect(index.subtypes, isEmpty);
   }
 
   test_subtypes_mixinDeclaration() async {
@@ -1095,17 +1084,15 @@
 mixin Y on A, B implements C;
 ''');
 
-    {
-      var X = index.subtypes.singleWhere((t) => t.name == 'X');
-      expect(X.supertypes, ['$libP;A', '$libP;B', '$libP;C']);
-      expect(X.members, isEmpty);
-    }
+    expect(index.supertypes, hasLength(6));
+    expect(index.subtypes, hasLength(6));
 
-    {
-      var Y = index.subtypes.singleWhere((t) => t.name == 'Y');
-      expect(Y.supertypes, ['$libP;A', '$libP;B', '$libP;C']);
-      expect(Y.members, isEmpty);
-    }
+    _assertSubtype(0, '$libP;A', 'X', []);
+    _assertSubtype(1, '$libP;A', 'Y', []);
+    _assertSubtype(2, '$libP;B', 'X', []);
+    _assertSubtype(3, '$libP;B', 'Y', []);
+    _assertSubtype(4, '$libP;C', 'X', []);
+    _assertSubtype(5, '$libP;C', 'Y', []);
   }
 
   test_usedName_inLibraryIdentifier() async {
@@ -1213,6 +1200,14 @@
         'not found\n$element $expectedRelationKind at $expectedLocation');
   }
 
+  void _assertSubtype(
+      int i, String superEncoded, String subName, List<String> members) {
+    expect(index.strings[index.supertypes[i]], superEncoded);
+    var subtype = index.subtypes[i];
+    expect(index.strings[subtype.name], subName);
+    expect(_decodeStringList(subtype.members), members);
+  }
+
   void _assertUsedName(String name, IndexRelationKind kind,
       ExpectedLocation expectedLocation, bool isNot) {
     int nameId = _getStringId(name);
@@ -1233,6 +1228,10 @@
     _failWithIndexDump('Not found $name $kind at $expectedLocation');
   }
 
+  List<String> _decodeStringList(List<int> stringIds) {
+    return stringIds.map((i) => index.strings[i]).toList();
+  }
+
   ExpectedLocation _expectedLocation(String search, bool isQualified,
       {int length}) {
     int offset = findOffset(search);