Always treat [call] members as closures during tracing.

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

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of type_graph_inferrer;
 
 // A set of selectors we know do not escape the elements inside the
 // list.
 Set<String> doesNotEscapeListSet = new Set<String>.from(
   const <String>[
@@ skipping 315 matching lines @@
     if (!element.isParameter()) return false;
     if (element.getEnclosingClass() != compiler.backend.mapImplementation) {
       return false;
     }
     Element method = element.enclosingElement;
     return (method.name == '[]=');
   }
 
   bool isClosure(Element element) {
     if (!element.isFunction()) return false;
+    /// Creating an instance of a class that implements [Function] also
+    /// closurizes the corresponding [call] member. We do not currently
+    /// track these, thus the check for [isClosurized] on such a method will
+    /// return false. Instead we catch that case here for now.
+    // TODO(herhut): Handle creation of closures from instances of Function.
+    if (element.isInstanceMember() &&
+        element.name == Compiler.CALL_OPERATOR_NAME) {
+      return true;
+    }
     Element outermost = element.getOutermostEnclosingMemberOrTopLevel();
     return outermost.declaration != element.declaration;
   }
 
   void visitElementTypeInformation(ElementTypeInformation info) {
     Element element = info.element;
     if (element.isParameter()
         && inferrer.isNativeElement(element.enclosingElement)) {
       bailout('Passed to a native method');
     }
     if (info.isClosurized()) {
       bailout('Returned from a closurized method');
     }
     if (isClosure(info.element)) {
       bailout('Returned from a closure');
     }
     if (compiler.backend.isNeededForReflection(info.element)) {
       bailout('Escape in reflection');
     }
     if (isParameterOfListAddingMethod(info.element) ||
         isParameterOfMapAddingMethod(info.element)) {
       // These elements are being handled in
       // [visitDynamicCallSiteTypeInformation].
       return;
     }
     addNewEscapeInformation(info);
   }
 }