Extract DartTypeVisitor and subclasses from resolution_types.

tests/compiler/dart2js/type_test_helper.dart

Index: tests/compiler/dart2js/type_test_helper.dart
diff --git a/tests/compiler/dart2js/type_test_helper.dart b/tests/compiler/dart2js/type_test_helper.dart
index 10a5ceaea119f65347ba12d2f32b2430ff9e314f..7d03616225bf5cac810e6b09a26a9e7f06938078 100644
--- a/tests/compiler/dart2js/type_test_helper.dart
+++ b/tests/compiler/dart2js/type_test_helper.dart
@@ -31,6 +31,8 @@ class TypeEnvironment {
 
   Resolution get resolution => compiler.resolution;
 
+  Types get types => resolution.types;
+
   static Future<TypeEnvironment> create(String source,
       {bool useMockCompiler: true,
       bool expectNoErrors: false,
@@ -113,20 +115,20 @@ class TypeEnvironment {
   }
 
   bool isSubtype(ResolutionDartType T, ResolutionDartType S) {
-    return compiler.types.isSubtype(T, S);
+    return types.isSubtype(T, S);
   }
 
   bool isMoreSpecific(ResolutionDartType T, ResolutionDartType S) {
-    return compiler.types.isMoreSpecific(T, S);
+    return types.isMoreSpecific(T, S);
   }
 
   ResolutionDartType computeLeastUpperBound(
       ResolutionDartType T, ResolutionDartType S) {
-    return compiler.types.computeLeastUpperBound(T, S);
+    return types.computeLeastUpperBound(T, S);
   }
 
   ResolutionDartType flatten(ResolutionDartType T) {
-    return compiler.types.flatten(T);
+    return types.flatten(T);
   }
 
   ResolutionFunctionType functionType(