Fix a bug where we would emit a noSuchMethod handler in the Object class for Object methods because…

sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart	(revision 26305)
+++ sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart	(working copy)
@@ -2818,6 +2818,11 @@
         TypeMask mask = selector.mask;
         if (mask == null) {
           mask = new TypeMask.subclass(compiler.objectClass.rawType);
+        } else if (mask.isEmpty && !mask.isNullable) {

[ahe, 2013/08/20 08:00:33]

I'm a little concerned about this. Perhaps it is just a sign that I don't fully
understand the type inferrer.

We saw this bug in code using mirrors which means that type inferencing is
disabled. That makes me wonder if this is a symptom fix for something else.

Also, if this kind of type is meaningless, why is it inferred?

[ngeoffray, 2013/08/20 08:03:48]

The bug is not necessarily related to type inferencing. The SSA backend can also
create these selectors with empty receiver type. It just happens that it does
not do anything with it and registers  the selector to the world. Maybe that's
the point where you would want the filtering to happen?

I think it belongs in this file, filtering earlier seems more like an
optimization.

[ahe, 2013/08/20 08:11:13]

I don't understand why it would be an optimization. What is the contract between
providers of type masks and the consumers of type masks? How do we guarantee
correctness?

I would say it is reasonable for the consumer (emitter in this case) to assume
the provider (SSA backend/inferrer) only produce valid type masks.

Otherwise, you have to check all places where you consume a typemask that it
isn't bogus.

[ngeoffray, 2013/08/20 08:24:03]

Why does it have to be bogus? There's an optimization phase that realizes the
type of an instruction is empty (take for example, a = 42; if (a is List)
a.toString();). If we don't do dead code elimination, the codegen will see this
call to a.toString, type the selector with a's type and register it to the
world. Checking if it's an empty selector seems like an optimization to me.

[ahe, 2013/08/20 08:31:44]

Your 
Let's talk about this face to face. Perhaps we need a whiteboard. I'm having a
really hard time understanding lines 2813-2877 (except, I understand lines
2872-2875).

[pavelj, 2013/08/20 14:51:44]

Peter, that version was not using mirrors.

On 2013/08/20 08:00:33, ahe wrote:

+          // We know a non-null empty mask cannot happen at runtime.
+          // We may have those selectors in the set of seen selectors
+          // if the backend did not optimize them.
+          continue;
         }
 
         // If the receiver is guaranteed to have a member that