Introduce DartType super hierarchy based on Entity

pkg/compiler/lib/src/ssa/codegen.dart

Index: pkg/compiler/lib/src/ssa/codegen.dart
diff --git a/pkg/compiler/lib/src/ssa/codegen.dart b/pkg/compiler/lib/src/ssa/codegen.dart
index 67bcd302144a64bf6072599f27bd6a43255a9bd9..2b3d5d49ac71a822ebbb4bc022b64ed50d4e6081 100644
--- a/pkg/compiler/lib/src/ssa/codegen.dart
+++ b/pkg/compiler/lib/src/ssa/codegen.dart
@@ -3003,7 +3003,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
       case TypeInfoExpressionKind.INSTANCE:
         // We expect only flat types for the INSTANCE representation.
         assert(
-            node.dartType == (node.dartType.element as ClassElement).thisType);
+            node.dartType == (node.dartType as InterfaceType).element.thisType);
         registry.registerInstantiatedClass(commonElements.listClass);
         push(new js.ArrayInitializer(arguments)
             .withSourceInformation(node.sourceInformation));
@@ -3071,8 +3071,8 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
       use(type);
       typeArguments.add(pop());
     }
-
-    ClassElement cls = node.dartType.element;
+    InterfaceType type = node.dartType;
+    ClassElement cls = type.element;
     var arguments = [backend.emitter.typeAccess(cls)];
     if (!typeArguments.isEmpty) {
       arguments.add(new js.ArrayInitializer(typeArguments));