Abstract over the type system.

pkg/analyzer/lib/src/context/context.dart

Index: pkg/analyzer/lib/src/context/context.dart
diff --git a/pkg/analyzer/lib/src/context/context.dart b/pkg/analyzer/lib/src/context/context.dart
index d90b0918cd0338597fce1088f24ce85a9fe490f6..86c81a67966abd62f645772ee039e2f511bc9262 100644
--- a/pkg/analyzer/lib/src/context/context.dart
+++ b/pkg/analyzer/lib/src/context/context.dart
@@ -161,6 +161,11 @@ class AnalysisContextImpl implements InternalAnalysisContext {
   TypeProvider _typeProvider;
 
   /**
+   * The [TypeSystem] for this context, `null` if not yet created.
+   */
+  TypeSystem _typeSystem;
+
+  /**
    * The controller for sending [SourcesChangedEvent]s.
    */
   StreamController<SourcesChangedEvent> _onSourcesChangedController;
@@ -479,6 +484,14 @@ class AnalysisContextImpl implements InternalAnalysisContext {
   }
 
   @override
+  TypeSystem get typeSystem {
+    if (_typeSystem == null) {
+      _typeSystem = new TypeSystemImpl(typeProvider);
+    }
+    return _typeSystem;
+  }
+
+  @override
   void addListener(AnalysisListener listener) {
     if (!_listeners.contains(listener)) {
       _listeners.add(listener);