dart2js: Keep refined type for type checks

pkg/compiler/lib/src/ssa/codegen_helpers.dart

Index: pkg/compiler/lib/src/ssa/codegen_helpers.dart
diff --git a/pkg/compiler/lib/src/ssa/codegen_helpers.dart b/pkg/compiler/lib/src/ssa/codegen_helpers.dart
index 9773af6306c13ecd3e660f73d05b6b6d748c274c..bf042e4c607e663b9c6ed6154bb2b43e14650c8b 100644
--- a/pkg/compiler/lib/src/ssa/codegen_helpers.dart
+++ b/pkg/compiler/lib/src/ssa/codegen_helpers.dart
@@ -283,6 +283,11 @@ class SsaTypeKnownRemover extends HBaseVisitor {
   }
 
   void visitTypeKnown(HTypeKnown instruction) {
+    for (HInstruction user in instruction.usedBy) {
+      if (user is HTypeConversion) {
+        user.inputType = instruction.instructionType;
+      }
+    }
     instruction.block.rewrite(instruction, instruction.checkedInput);
     instruction.block.remove(instruction);
   }