Move ElementEnvironment.isSubtype to DartTypes

pkg/compiler/lib/src/common_elements.dart

Index: pkg/compiler/lib/src/common_elements.dart
diff --git a/pkg/compiler/lib/src/common_elements.dart b/pkg/compiler/lib/src/common_elements.dart
index 64d71fd199ea4684a479eedcf0a6218cd31abce4..205dbb7a2cee7b83188c3755544bee86a6f561e8 100644
--- a/pkg/compiler/lib/src/common_elements.dart
+++ b/pkg/compiler/lib/src/common_elements.dart
@@ -1246,9 +1246,6 @@ abstract class ElementEnvironment {
   /// `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);