Move ElementEnvironment.isSubtype to DartTypes

pkg/compiler/lib/src/common_elements.dart

 // Copyright (c) 2014, 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.
 
 // TODO(sigmund): rename and move to common/elements.dart
 library dart2js.type_system;
 
 import 'common/names.dart' show Identifiers, Uris;
 import 'constants/values.dart';
 import 'elements/entities.dart';
@@ skipping 1228 matching lines @@
   /// where the type arguments are the type variables of [cls].
   InterfaceType getThisType(ClassEntity cls);
 
   /// Returns `true` if [cls] is generic.
   bool isGenericClass(ClassEntity cls);
 
   /// The upper bound on the [typeVariable]. If not explicitly declared, this is
   /// `Object`.
   DartType getTypeVariableBound(TypeVariableEntity typeVariable);
 
-  /// Returns `true` if [a] is a subtype of [b].
-  bool isSubtype(DartType a, DartType b);
-
   /// Returns the type if [function].
   FunctionType getFunctionType(FunctionEntity function);
 
   /// Returns the type of the [local] function.
   FunctionType getLocalFunctionType(Local local);
 
   /// Returns the unaliased type of [type].
   ///
   /// Use this during resolution to ensure that the alias has been computed.
   // TODO(johnniwinther): Remove this when the resolver is removed.
   DartType getUnaliasedType(DartType type);
 
   /// Returns the [CallStructure] corresponding to calling [entity] with all
   /// arguments, both required and optional.
   CallStructure getCallStructure(FunctionEntity entity);
 
   /// Returns `true` if [member] a the synthetic getter `loadLibrary` injected
   /// on deferred libraries.
   bool isDeferredLoadLibraryGetter(MemberEntity member);
 
   /// Returns the metadata constants declared on [member].
   Iterable<ConstantValue> getMemberMetadata(MemberEntity member);
 }