Support runtime check of function types.

tests/compiler/dart2js/subtype_test.dart

Index: tests/compiler/dart2js/subtype_test.dart
diff --git a/tests/compiler/dart2js/subtype_test.dart b/tests/compiler/dart2js/subtype_test.dart
index b4efe86a379b780bdadbe551b75fd428b963defd..35a46517582f585d7092c458cbe068ef433df522 100644
--- a/tests/compiler/dart2js/subtype_test.dart
+++ b/tests/compiler/dart2js/subtype_test.dart
@@ -264,6 +264,7 @@ void testCallableSubtype() {
 
 testFunctionSubtyping() {
   var env = new TypeEnvironment(r"""
+      _() => null;
       void void_() {}
       void void_2() {}
       int int_() => 0;
@@ -285,6 +286,7 @@ testFunctionSubtyping() {
 
 testTypedefSubtyping() {
   var env = new TypeEnvironment(r"""
+      typedef _();
       typedef void void_();
       typedef void void_2();
       typedef int int_();
@@ -317,6 +319,13 @@ functionSubtypingHelper(TypeEnvironment env) {
   // Function <: () -> int
   expect(false, 'Function', 'int_');
 
+  // () -> dynamic <: () -> dynamic
+  expect(true, '_', '_');
+  // () -> dynamic <: () -> void
+  expect(true, '_', 'void_');
+  // () -> void <: () -> dynamic
+  expect(true, 'void_', '_');
+
   // () -> int <: () -> void
   expect(true, 'int_', 'void_');
   // () -> void <: () -> int