Only emit typeguards if we think they are valuable.

lib/compiler/implementation/ssa/nodes.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 interface HVisitor<R> {
   R visitAdd(HAdd node);
   R visitBitAnd(HBitAnd node);
   R visitBitNot(HBitNot node);
   R visitBitOr(HBitOr node);
   R visitBitXor(HBitXor node);
@@ skipping 427 matching lines @@
 
   int hashCode() => id;
 
   bool isNew() => status == STATUS_NEW;
   bool isOpen() => status == STATUS_OPEN;
   bool isClosed() => status == STATUS_CLOSED;
 
   bool isLoopHeader() => blockInformation is HLoopInformation;
   bool isLabeledBlock() => blockInformation is HLabeledBlockInformation;
 
+  HBasicBlock get enclosingLoopHeader() {
+    if (isLoopHeader()) return this;
+    return parentLoopHeader;
+  }
+
   bool hasGuards() => !guards.isEmpty();
 
   void open() {
     assert(isNew());
     status = STATUS_OPEN;
   }
 
   void close(HControlFlow end) {
     assert(isOpen());
     addAfter(last, end);
@@ skipping 1841 matching lines @@
   final bool isAnd;
   final SubExpression left;
   final SubExpression right;
   final HBasicBlock joinBlock;
   HAndOrBlockInformation(this.isAnd,
                          this.left,
                          this.right,
                          this.joinBlock);
   bool accept(HBlockInformationVisitor visitor) => visitor.visitAndOrInfo(this);
 }