Postpone the argument type optimizations until the types are stable.

lib/compiler/implementation/ssa/codegen_helpers.dart

Index: lib/compiler/implementation/ssa/codegen_helpers.dart
diff --git a/lib/compiler/implementation/ssa/codegen_helpers.dart b/lib/compiler/implementation/ssa/codegen_helpers.dart
index f1b5fa7747187cd0ba3ef1213abebed835aec021..cdc2864ed9a3f2926fa1da084adc26b781e34ad9 100644
--- a/lib/compiler/implementation/ssa/codegen_helpers.dart
+++ b/lib/compiler/implementation/ssa/codegen_helpers.dart
@@ -52,7 +52,7 @@ class SsaInstructionMerger extends HBaseVisitor {
       generateAtUseSite.add(instruction);
     } else if (instruction.isCheckedModeCheck()) {
       // Checked mode checks compile to code that only use their input
-      // once, so we can safely visit them an try to merge the input.
+      // once, so we can safely visit them and try to merge the input.
       visitInstruction(instruction);
     }
   }