Context objects don't have a compile-type. Return dynamic-type in this case.

runtime/vm/flow_graph_type_propagator.cc

Index: runtime/vm/flow_graph_type_propagator.cc
===================================================================
--- runtime/vm/flow_graph_type_propagator.cc	(revision 41396)
+++ runtime/vm/flow_graph_type_propagator.cc	(working copy)
@@ -548,9 +548,9 @@
   if (type_ == NULL) {
     ASSERT(cid_ != kIllegalCid);
 
-    // VM internal Function objects don't have a compile-type. Return
-    // dynamic-type in this case.
-    if (cid_ == kFunctionCid) {
+    // VM internal Function and Context objects don't have a compile-type.
+    // Return dynamic-type in this case.
+    if (cid_ == kFunctionCid || cid_ == kContextCid) {
       type_ = &Type::ZoneHandle(Type::DynamicType());
       return type_;
     }