Fix issue 12336: there was a bug when updating phis at exit of a block with a HTypeKnown instructio…

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

Index: sdk/lib/_internal/compiler/implementation/ssa/codegen.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/ssa/codegen.dart	(revision 29323)
+++ sdk/lib/_internal/compiler/implementation/ssa/codegen.dart	(working copy)
@@ -608,6 +608,8 @@
       visitExpression(argument);
     } else if (argument is HCheck && !variableNames.hasName(argument)) {
       HCheck check = argument;
+      // This can only happen if the checked node does not have a name.
+      assert(!variableNames.hasName(check.checkedInput));
       use(check.checkedInput);
     } else {
       assert(variableNames.hasName(argument));