Use closure tracer to identify closures that are not passed to Function.apply

sdk/lib/_internal/compiler/implementation/world.dart

 // Copyright (c) 2012, 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 dart2js;
 
 class World {
   final Compiler compiler;
   final FunctionSet allFunctions;
   final Set<Element> functionsCalledInLoop = new Set<Element>();
@@ skipping 11 matching lines @@
       new Map<ClassElement, Set<ClassElement>>();
   final Map<ClassElement, Set<ClassElement>> _subtypes =
       new Map<ClassElement, Set<ClassElement>>();
   final Map<ClassElement, Set<ClassElement>> _supertypes =
       new Map<ClassElement, Set<ClassElement>>();
 
   final Set<Element> sideEffectsFreeElements = new Set<Element>();
 
   final Set<Element> elementsThatCannotThrow = new Set<Element>();
 
+  final Set<Element> functionsThatMightBePassedToApply =
+      new Set<FunctionElement>();
+
   Set<ClassElement> subclassesOf(ClassElement cls) {
     return _subclasses[cls.declaration];
   }
 
   Set<ClassElement> subtypesOf(ClassElement cls) {
     return _subtypes[cls.declaration];
   }
 
   Set<ClassElement> supertypesOf(ClassElement cls) {
     return _supertypes[cls.declaration];
@@ skipping 229 matching lines @@
   }
 
   bool getCannotThrow(Element element) {
     return elementsThatCannotThrow.contains(element);
   }
 
   void registerImplicitSuperCall(TreeElements elements,
                                  FunctionElement superConstructor) {
     elements.otherDependencies.add(superConstructor);
   }
+
+  void registerMightBePassedToApply(Element element) {
+    functionsThatMightBePassedToApply.add(element);
+  }
+
+  bool getMightBePassedToApply(Element element) {
+    // We have to check whether the element we look at was created after
+    // type inference ran. This is currently only the case for the call
+    // method of function classes that were generated for function
+    // expressions. In such a case, we have to look at the original
+    // function expressions's element.
+    // TODO(herhut): Generate classes for function expressions earlier.
+    if (element is SynthesizedCallMethodElementX) {
+      return getMightBePassedToApply(element.expression);
+    }
+    return functionsThatMightBePassedToApply.contains(element);
+  }
 }