Improve checked mode a little (more to come in a different CL).

runtime/vm/code_generator_ia32.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/globals.h"  // Needed here to get TARGET_ARCH_IA32.
 #if defined(TARGET_ARCH_IA32)
 
 #include "vm/code_generator.h"
 
 #include "lib/error.h"
@@ skipping 687 matching lines @@
     } else {
       __ LoadObject(EAX, literal);
     }
   } else {
     // Pop the previously evaluated result value into EAX.
     __ popl(EAX);
   }
 
   // Generate type check.
   if (FLAG_enable_type_checks) {
+    const bool returns_null = node->value()->IsLiteralNode() &&
+       node->value()->AsLiteralNode()->literal().IsNull();
     const RawFunction::Kind  kind = parsed_function().function().kind();
     // Implicit getters do not need a type check at return.
-    if ((kind != RawFunction::kImplicitGetter) &&
+    if (!returns_null &&
+        (kind != RawFunction::kImplicitGetter) &&
         (kind != RawFunction::kConstImplicitGetter)) {
       GenerateAssertAssignable(
           node->id(),
           node->value()->token_index(),
           AbstractType::ZoneHandle(parsed_function().function().result_type()),
           String::ZoneHandle(String::NewSymbol("function result")));
     }
   }
   GenerateReturnEpilog(node);
 }
@@ skipping 1976 matching lines @@
   const Error& error = Error::Handle(
       Parser::FormatError(script, token_index, "Error", format, args));
   va_end(args);
   Isolate::Current()->long_jump_base()->Jump(1, error);
   UNREACHABLE();
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_IA32