Fix issue 9228, by nopt doing some optimizations in checked mode.

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

Index: sdk/lib/_internal/compiler/implementation/ssa/bailout.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/ssa/bailout.dart	(revision 23244)
+++ sdk/lib/_internal/compiler/implementation/ssa/bailout.dart	(working copy)
@@ -295,7 +295,10 @@
         receiverSelectorOnThrow = selector;
         willThrow = true;
       }
-    } else if (willThrowArgumentError(selector, receiver, speculativeType)) {
+    // We need to call the actual method in checked mode to get
+    // the right type error.
+    } else if (!compiler.enableTypeAssertions
+               && willThrowArgumentError(selector, receiver, speculativeType)) {
       willThrow = true;
     }