Fix narrowing for null checks in type graph inferrer.

sdk/lib/_internal/compiler/implementation/inferrer/type_graph_inferrer.dart

Index: sdk/lib/_internal/compiler/implementation/inferrer/type_graph_inferrer.dart
diff --git a/sdk/lib/_internal/compiler/implementation/inferrer/type_graph_inferrer.dart b/sdk/lib/_internal/compiler/implementation/inferrer/type_graph_inferrer.dart
index d7375f66a2b2358fd66003d7bf6c797eb2ec6d2e..caadbaca245bac1c08a31a477600dbcddd378bc6 100644
--- a/sdk/lib/_internal/compiler/implementation/inferrer/type_graph_inferrer.dart
+++ b/sdk/lib/_internal/compiler/implementation/inferrer/type_graph_inferrer.dart
@@ -213,7 +213,7 @@ class TypeInformationSystem extends TypeSystem<TypeInformation> {
                              {bool isNullable: true}) {
     if (annotation.treatAsDynamic) return type;
     if (annotation.isVoid) return nullType;
-    if (annotation.element == compiler.objectClass) return type;
+    if (annotation.element == compiler.objectClass && isNullable) return type;
     TypeMask otherType;
     if (annotation.isTypedef || annotation.isFunctionType) {
       otherType = functionType.type;
@@ -222,7 +222,9 @@ class TypeInformationSystem extends TypeSystem<TypeInformation> {
       return type;
     } else {
       assert(annotation.isInterfaceType);
-      otherType = new TypeMask.nonNullSubtype(annotation.element);
+      otherType = annotation.element == compiler.objectClass
+          ? dynamicType.type.nonNullable()

[floitsch, 2014/08/15 17:37:44]

Why is this necessary?
Isn't that the same?

[herhut, 2014/08/18 08:06:51]

We can have [subclass object] and [subtype object], which contain the same set
of classes but we use the former for dynamic, whereas we would normally create
the latter for narrowing.
Unfortunately, we often use structural equality for type comparisons, where
subtype object != subclass object. In that sense, they are not the same.

+          : new TypeMask.nonNullSubtype(annotation.element);
     }
     if (isNullable) otherType = otherType.nullable();
     if (type.type.isExact) {