Add default type arguments on Supertype as well.

pkg/front_end/lib/src/fasta/kernel/kernel_class_builder.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.
 
 library fasta.kernel_class_builder;
 
 import 'package:kernel/ast.dart'
     show
         Class,
         DartType,
@@ skipping 46 matching lines @@
       : super(metadata, modifiers, name, typeVariables, supertype, interfaces,
             members, parent, charOffset);
 
   Class get cls;
 
   Class get target => cls;
 
   /// [arguments] have already been built.
   InterfaceType buildTypesWithBuiltArguments(
       LibraryBuilder library, List<DartType> arguments) {
-    return arguments == null
-        ? cls.rawType
-        : new InterfaceType(
-            cls,
-            // TODO(ahe): Not sure what to do if `arguments.length !=
-            // cls.typeParameters.length`.
-            computeDefaultTypeArguments(cls.typeParameters, arguments));
+    assert(arguments == null || cls.typeParameters.length == arguments.length);
+    return arguments == null ? cls.rawType : new InterfaceType(cls, arguments);
+  }
+
+  List<DartType> buildTypeArguments(
+      LibraryBuilder library, List<KernelTypeBuilder> arguments) {
+    List<DartType> typeArguments = <DartType>[];
+    for (KernelTypeBuilder builder in arguments) {
+      DartType type = builder.build(library);
+      if (type == null) {
+        internalError("Bad type: ${builder.runtimeType}");
+      }
+      typeArguments.add(type);
+    }
+    return computeDefaultTypeArguments(
+        library, cls.typeParameters, typeArguments);
   }
 
   InterfaceType buildType(
       LibraryBuilder library, List<KernelTypeBuilder> arguments) {
     List<DartType> typeArguments;
     if (arguments != null) {
-      typeArguments = <DartType>[];
-      for (KernelTypeBuilder builder in arguments) {
-        DartType type = builder.build(library);
-        if (type == null) {
-          internalError("Bad type: ${builder.runtimeType}");
-        }
-        typeArguments.add(type);
-      }
+      typeArguments = buildTypeArguments(library, arguments);
     }
     return buildTypesWithBuiltArguments(library, typeArguments);
   }
 
   Supertype buildSupertype(
       LibraryBuilder library, List<KernelTypeBuilder> arguments) {
-    List<DartType> typeArguments;
     if (arguments != null) {
-      typeArguments = <DartType>[];
-      for (KernelTypeBuilder builder in arguments) {
-        DartType type = builder.build(library);
-        if (type == null) {
-          internalError("Bad type: ${builder.runtimeType}");
-        }
-        typeArguments.add(type);
-      }
-      return new Supertype(cls, typeArguments);
+      return new Supertype(cls, buildTypeArguments(library, arguments));
     } else {
       return cls.asRawSupertype;
     }
   }
 
   int resolveConstructors(KernelLibraryBuilder library) {
     int count = super.resolveConstructors(library);
     if (count != 0) {
       // Copy keys to avoid concurrent modification error.
       List<String> names = members.keys.toList();
@@ skipping 52 matching lines @@
           fileUri: cls.fileUri)..fileOffset = cls.fileOffset;
       cls.addMember(field);
       return new DillMemberBuilder(field, this);
     });
     Field field = constructorsField.target;
     ListLiteral literal = field.initializer;
     literal.expressions
         .add(new StaticGet(constructor.target)..parent = literal);
   }
 }