Remove AbstractVisitor.

dart/lib/compiler/implementation/ssa/builder.dart

Index: dart/lib/compiler/implementation/ssa/builder.dart
diff --git a/dart/lib/compiler/implementation/ssa/builder.dart b/dart/lib/compiler/implementation/ssa/builder.dart
index 5e58525d4889c828a1975eb61779ab63eb1b4e5c..bec5660de2e6603e28213ad45e0954e333b60694 100644
--- a/dart/lib/compiler/implementation/ssa/builder.dart
+++ b/dart/lib/compiler/implementation/ssa/builder.dart
@@ -4013,7 +4013,7 @@ class SsaBuilder extends ResolvedVisitor implements Visitor {
  * TODO(lrn): Consider whether to handle compile time constant int/boolean
  * expressions as well.
  */
-class StringBuilderVisitor extends AbstractVisitor {
+class StringBuilderVisitor extends Visitor {
   final SsaBuilder builder;
   final Node diagnosticNode;
 
@@ -4066,7 +4066,7 @@ class StringBuilderVisitor extends AbstractVisitor {
  * This class visits the method that is a candidate for inlining and
  * finds whether it is too difficult to inline.
  */
-class InlineWeeder extends AbstractVisitor {
+class InlineWeeder extends Visitor {
   final TreeElements elements;
   bool seenReturn = false;
   bool tooDifficult = false;