Support general handling of type variables by substituting types into the current context.

sdk/lib/_internal/compiler/implementation/elements/modelx.dart

Index: sdk/lib/_internal/compiler/implementation/elements/modelx.dart
diff --git a/sdk/lib/_internal/compiler/implementation/elements/modelx.dart b/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
index 11eeb0106458a65e0ba8ee8d745ee91b78dbbde4..57b257c931869cd384bb8aa6b34efe31f71b998b 100644
--- a/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
+++ b/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
@@ -225,6 +225,18 @@ abstract class ElementX implements Element {
     return null;
   }
 
+  ClassElement get contextClass {
+    ClassElement cls;
+    for (Element e = this; e != null; e = e.enclosingElement) {
+      if (e.isClass) {
+        // Record [e] instead of returning it directly. We need the last class
+        // in the chain since the first classes might be closure classes.
+        cls = e.declaration;
+      }
+    }
+    return cls;
+  }
+
   /**
    * Creates the scope for this element.
    */
@@ -1917,6 +1929,11 @@ abstract class BaseClassElementX extends ElementX
 
   Link<DartType> computeTypeParameters(Compiler compiler);
 
+  InterfaceType asInstanceOf(ClassElement cls) {
+    if (cls == this) return thisType;
+    return allSupertypesAndSelf.asInstanceOf(cls);
+  }
+
   /**
    * Return [:true:] if this element is the [:Object:] class for the [compiler].
    */