Implementation of null-aware operators.

pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart

Index: pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
diff --git a/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart b/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
index 2b63c0c7d61c5e7939722eac8b3cebaae9f6f773..f13911a702765a61a054a78285944d458402c893 100644
--- a/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
+++ b/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
@@ -27,6 +27,8 @@ import '../native/native.dart' as native;
 import '../tree/tree.dart' as ast
     show DartString,
          Node,
+         Send,
+         SendSet,
          TryStatement;
 import '../types/types.dart'
     show ContainerTypeMask,
@@ -248,9 +250,13 @@ class TypeInformationSystem extends TypeSystem<TypeInformation> {
     return info.type != selector.mask;
   }
 
-  TypeInformation refineReceiver(Selector selector, TypeInformation receiver) {
+  TypeInformation refineReceiver(Selector selector, TypeInformation receiver,
+      bool isConditional) {
     if (receiver.type.isExact) return receiver;
     TypeMask otherType = compiler.world.allFunctions.receiverType(selector);
+    // Conditional sends (a?.b) can still narrow the possible types of `a`,
+    // however, we still need to consider that `a` may be null.
+    if (isConditional) otherType = otherType.nullable();

[sra1, 2015/05/22 21:17:28]

Only do this if receiver type is nullable.
"Hello".toUpperCase() should not be inferred nullable.

  if (isConditional && receiver.type.isNullable)

This will become increasingly important as people get lazy and use ?.
everywhere.

[Siggi Cherem (dart-lang), 2015/05/22 22:36:08]

Done.

     // If this is refining to nullable subtype of `Object` just return
     // the receiver. We know the narrowing is useless.
     if (otherType.isNullable && otherType.containsAll(classWorld)) {