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

Index: sdk/lib/_internal/compiler/implementation/ssa/nodes.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/ssa/nodes.dart	(revision 19109)
+++ sdk/lib/_internal/compiler/implementation/ssa/nodes.dart	(working copy)
@@ -1217,6 +1217,12 @@
   bool isJsStatement() => isEnabled;
   bool canThrow() => isEnabled;
 
+  // A [HTypeGuard] cannot be moved anywhere in the graph, otherwise
+  // instructions that have side effects could end up before the guard
+  // in the otpimized version, and after the guard in a bailout
+  // version.
+  bool isPure() => false;
+
   accept(HVisitor visitor) => visitor.visitTypeGuard(this);
   int typeCode() => HInstruction.TYPE_GUARD_TYPECODE;
   bool typeEquals(other) => other is HTypeGuard;