Fix for issue 8781: it is not ok to move a HTypeGuard, so mark it as "unpure".

sdk/lib/_internal/compiler/implementation/ssa/nodes.dart

 // Copyright (c) 2012, 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.
 
 part of ssa;
 
 abstract class HVisitor<R> {
   R visitAdd(HAdd node);
   R visitBailoutTarget(HBailoutTarget node);
   R visitBitAnd(HBitAnd node);
@@ skipping 1198 matching lines @@
   }
 
   void disable() {
     isEnabled = false;
     instructionType = guarded.instructionType;
   }
 
   bool isControlFlow() => true;
   bool isJsStatement() => isEnabled;
   bool canThrow() => isEnabled;
+  // A [HTypeGuard] cannot be moved anywhere in the graph.

[kasperl, 2013/02/26 17:36:48]

I would add a newline before the comment and extend the comment with information
about why the guard cannot be moved.

[ngeoffray, 2013/02/27 08:06:54]

Done.

+  bool isPure() => false;
 
   accept(HVisitor visitor) => visitor.visitTypeGuard(this);
   int typeCode() => HInstruction.TYPE_GUARD_TYPECODE;
   bool typeEquals(other) => other is HTypeGuard;
   bool dataEquals(HTypeGuard other) => guardedType == other.guardedType;
 }
 
 class HBoundsCheck extends HCheck {
   static const int ALWAYS_FALSE = 0;
   static const int FULL_CHECK = 1;
@@ skipping 1327 matching lines @@
   HBasicBlock get start => expression.start;
   HBasicBlock get end {
     // We don't create a switch block if there are no cases.
     assert(!statements.isEmpty);
     return statements.last.end;
   }
 
   bool accept(HStatementInformationVisitor visitor) =>
       visitor.visitSwitchInfo(this);
 }