Implementation of null-aware operators.

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

Index: pkg/compiler/lib/src/inferrer/type_graph_nodes.dart
diff --git a/pkg/compiler/lib/src/inferrer/type_graph_nodes.dart b/pkg/compiler/lib/src/inferrer/type_graph_nodes.dart
index a12b342145f15d65d25da6cbb76c312ce0f64a07..c77f169cb1e1a9ac6fe275f830364bc8822738e3 100644
--- a/pkg/compiler/lib/src/inferrer/type_graph_nodes.dart
+++ b/pkg/compiler/lib/src/inferrer/type_graph_nodes.dart
@@ -897,7 +897,7 @@ class DynamicCallSiteTypeInformation extends CallSiteTypeInformation {
 
     // Walk over the found targets, and compute the joined union type mask
     // for all these targets.
-    return inferrer.types.joinTypeMasks(targets.map((element) {
+    TypeMask result = inferrer.types.joinTypeMasks(targets.map((element) {
       // If [canReachAll] is true, then we are iterating over all
       // targets that satisfy the untyped selector. We skip the return
       // type of the targets that can only be reached through
@@ -942,6 +942,15 @@ class DynamicCallSiteTypeInformation extends CallSiteTypeInformation {
         return inferrer.typeOfElementWithSelector(element, typedSelector).type;
       }
     }));
+
+    if (call is ast.Send) {
+      Send send = call;
+      if (send.isConditional && receiver.type.isNullable) {
+        // Conditional sends (e.g. `a?.b`) may be null if the receiver is null.
+        result = result.nullable();
+      }
+    }
+    return result;
   }
 
   void giveUp(TypeGraphInferrerEngine inferrer, {bool clearAssignments: true}) {