Improve null-check elimination and change backward traversal strategy.

pkg/compiler/lib/src/cps_ir/backward_null_check_remover.dart

Index: pkg/compiler/lib/src/cps_ir/backward_null_check_remover.dart
diff --git a/pkg/compiler/lib/src/cps_ir/backward_null_check_remover.dart b/pkg/compiler/lib/src/cps_ir/backward_null_check_remover.dart
index e5caa8342626ccc6aab96f1c667d957c5ad32605..8a941e70c6aa832bb9e3494b978dae051ce47f26 100644
--- a/pkg/compiler/lib/src/cps_ir/backward_null_check_remover.dart
+++ b/pkg/compiler/lib/src/cps_ir/backward_null_check_remover.dart
@@ -31,8 +31,7 @@ import 'cps_fragment.dart';
 //   bad so changing that should be ok, but changing a field access is not as
 //   clear.
 //
-class BackwardNullCheckRemover extends TrampolineRecursiveVisitor
-                               implements Pass {
+class BackwardNullCheckRemover extends BlockVisitor implements Pass {
   String get passName => 'Backward null-check remover';
 
   final TypeMaskSystem typeSystem;
@@ -41,10 +40,14 @@ class BackwardNullCheckRemover extends TrampolineRecursiveVisitor
   /// a value that is checked in the beginning of that expression.
   Primitive nullCheckedValue;
 
+  /// The [nullCheckedValue] at the entry point of a continuation.
+  final Map<Continuation, Primitive> nullCheckedValueAt =
+      <Continuation, Primitive>{};
+
   BackwardNullCheckRemover(this.typeSystem);
 
   void rewrite(FunctionDefinition node) {
-    visit(node);
+    BlockVisitor.traverseInPostOrder(node, this);
   }
 
   /// Returns a reference to an operand of [prim], where [prim] throws if null
@@ -95,37 +98,33 @@ class BackwardNullCheckRemover extends TrampolineRecursiveVisitor
     return prim.isSafeForReordering;
   }
 
-  Expression traverseLetPrim(LetPrim node) {
+  void visitLetPrim(LetPrim node) {
     Primitive prim = node.primitive;
     Primitive receiver = getNullCheckedOperand(prim)?.definition;
     if (receiver != null) {
-      pushAction(() {
-        Primitive successor = nullCheckedValue;
-        if (successor != null && receiver.sameValue(successor)) {
-          tryEliminateRedundantNullCheck(prim, successor);
-        }
-        nullCheckedValue = receiver;
-      });
+      if (nullCheckedValue != null && receiver.sameValue(nullCheckedValue)) {
+        tryEliminateRedundantNullCheck(prim, nullCheckedValue);
+      }
+      nullCheckedValue = receiver;
     } else if (!canMoveAboveNullCheck(prim)) {
-      pushAction(() {
-        nullCheckedValue = null;
-      });
+      nullCheckedValue = null;
     }
-    return node.body;
   }
 
-  Expression traverseContinuation(Continuation cont) {
-    pushAction(() {
+  void visitContinuation(Continuation cont) {
+    if (nullCheckedValue != null) {
+      nullCheckedValueAt[cont] = nullCheckedValue;
       nullCheckedValue = null;
-    });
-    return cont.body;
+    }
   }
 
-  Expression traverseLetHandler(LetHandler node) {
-    push(node.handler);
-    pushAction(() {
-      nullCheckedValue = null;
-    });
-    return node.body;
+  void visitLetHandler(LetHandler node) {
+    nullCheckedValue = null;
+  }
+
+  visitInvokeContinuation(InvokeContinuation node) {
+    if (!node.isRecursive) {
+      nullCheckedValue = nullCheckedValueAt[node.continuation.definition];
+    }
   }
 }