Implement references to "this" in the new IR.

sdk/lib/_internal/compiler/implementation/dart_backend/dart_codegen.dart

Index: sdk/lib/_internal/compiler/implementation/dart_backend/dart_codegen.dart
diff --git a/sdk/lib/_internal/compiler/implementation/dart_backend/dart_codegen.dart b/sdk/lib/_internal/compiler/implementation/dart_backend/dart_codegen.dart
index 97cc8cadb152afb60ff0c4f033576b817e23c236..4906aaaaaf0a1b4780e0234bb492cd6977a940dc 100644
--- a/sdk/lib/_internal/compiler/implementation/dart_backend/dart_codegen.dart
+++ b/sdk/lib/_internal/compiler/implementation/dart_backend/dart_codegen.dart
@@ -333,6 +333,10 @@ class ASTEmitter extends tree.Visitor<dynamic, Expression> {
     return emitConstant(exp.value);
   }
 
+  Expression visitThis(tree.This exp) {
+    return new This();
+  }
+
   Expression visitLiteralList(tree.LiteralList exp) {
     return new LiteralList(
         exp.values.map(visitExpression).toList(growable: false),