After a dynamic call, refine the receiver type by looking at the potential targets of that call.

sdk/lib/_internal/compiler/implementation/ssa/types.dart

Index: sdk/lib/_internal/compiler/implementation/ssa/types.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/ssa/types.dart	(revision 22031)
+++ sdk/lib/_internal/compiler/implementation/ssa/types.dart	(working copy)
@@ -41,7 +41,9 @@
 
     // TODO(kasperl): A lot of the code in the system currently
     // expects the top type to be 'unknown'. I'll rework this.
-    if (element == compiler.objectClass || element == compiler.dynamicClass) {
+    if (mask.refined == null
+        && (element == compiler.objectClass
+            || element == compiler.dynamicClass)) {
       return isNullable ? HType.UNKNOWN : HType.NON_NULL;
     }