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/variable_allocator.dart

Index: sdk/lib/_internal/compiler/implementation/ssa/variable_allocator.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/ssa/variable_allocator.dart	(revision 29323)
+++ sdk/lib/_internal/compiler/implementation/ssa/variable_allocator.dart	(working copy)
@@ -682,6 +682,10 @@
     for (int i = 0; i < phi.inputs.length; i++) {
       HInstruction input = phi.inputs[i];
       HBasicBlock predecessor = phi.block.predecessors[i];
+      // A [HTypeKnown] instruction never has a name, but its checked
+      // input might, therefore we need to do a copy instead of an
+      // assignment.
+      while (input is HTypeKnown) input = input.inputs[0];
       if (!needsName(input)) {
         names.addAssignment(predecessor, input, phi);
       } else {