Extract DartTypeVisitor and subclasses from resolution_types.

pkg/compiler/lib/src/compiler.dart

Index: pkg/compiler/lib/src/compiler.dart
diff --git a/pkg/compiler/lib/src/compiler.dart b/pkg/compiler/lib/src/compiler.dart
index 47cbb8b8208b1d97d48700722740264286326d62..c52c3c877e38eb8eeb0afde920748cd6c1d22d1e 100644
--- a/pkg/compiler/lib/src/compiler.dart
+++ b/pkg/compiler/lib/src/compiler.dart
@@ -34,6 +34,7 @@ import 'elements/entities.dart';
 import 'elements/modelx.dart' show ErroneousElementX;
 import 'elements/resolution_types.dart'
     show
+        DartTypes,
         ResolutionDartType,
         ResolutionDynamicType,
         ResolutionFunctionType,
@@ -86,7 +87,7 @@ abstract class Compiler {
   Measurer get measurer;
 
   final IdGenerator idGenerator = new IdGenerator();
-  Types types;
+  DartTypes types;
   CommonElements _commonElements;
   _CompilerElementEnvironment _elementEnvironment;
   CompilerDiagnosticReporter _reporter;