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

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

Index: sdk/lib/_internal/compiler/implementation/world.dart
diff --git a/sdk/lib/_internal/compiler/implementation/world.dart b/sdk/lib/_internal/compiler/implementation/world.dart
index 033c95ba404981cbe512c2a03a31ccb36240b2d5..2d5accdb4e2a3725ff9c143d4600e998a3fef19f 100644
--- a/sdk/lib/_internal/compiler/implementation/world.dart
+++ b/sdk/lib/_internal/compiler/implementation/world.dart
@@ -29,6 +29,9 @@ class World {
 
   final Set<Element> elementsThatCannotThrow = new Set<Element>();
 
+  final Set<Element> functionsThatMightBePassedToApply =
+      new Set<FunctionElement>();
+
   Set<ClassElement> subclassesOf(ClassElement cls) {
     return _subclasses[cls.declaration];
   }
@@ -278,4 +281,21 @@ class World {
                                  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);
+  }
 }