Implement subtype relation for kernel based elements.

pkg/compiler/lib/src/elements/resolution_types.dart

Index: pkg/compiler/lib/src/elements/resolution_types.dart
diff --git a/pkg/compiler/lib/src/elements/resolution_types.dart b/pkg/compiler/lib/src/elements/resolution_types.dart
index 016f5c3edf4754269b6552472be56fbc12037c89..5ab0f2bb50f470a01a6077aa079b23c67a9dccf4 100644
--- a/pkg/compiler/lib/src/elements/resolution_types.dart
+++ b/pkg/compiler/lib/src/elements/resolution_types.dart
@@ -1001,7 +1001,6 @@ abstract class BaseResolutionDartTypeVisitor<R, A>
       visitGenericType(type, argument);
 }
 
-/// Abstract visitor for determining relations between types.
 abstract class AbstractTypeRelationMixin
     implements AbstractTypeRelation, ResolutionDartTypeVisitor<bool, DartType> {
   Resolution get resolution;
@@ -1072,22 +1071,6 @@ typedef void CheckTypeVariableBound(
     ResolutionTypeVariableType typeVariable,
     ResolutionDartType bound);
 
-/// Basic interface for the Dart type system.
-abstract class DartTypes {
-  /// The types defined in 'dart:core'.
-  CommonElements get commonElements;
-
-  /// Returns `true` if [t] is a subtype of [s].
-  bool isSubtype(DartType t, DartType s);
-
-  /// Returns `true` if [t] is assignable to [s].
-  bool isAssignable(DartType t, DartType s);
-
-  /// Returns `true` if [t] might be a subtype of [s] for some values of
-  /// type variables in [s] and [t].
-  bool isPotentialSubtype(DartType t, DartType s);
-}
-
 class Types implements DartTypes {
   final Resolution resolution;
   final ResolutionMoreSpecificVisitor moreSpecificVisitor;