Upgrade some tests to use annotations instead of 'clever' confuse() function

tests/language/pure_function2_test.dart

 // Copyright (c) 2014, 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.
 
 import "package:expect/expect.dart";
 
 // Regression test for issue 17483.
 
-confuse(x) {
-  if (new DateTime.now().millisecondsSinceEpoch == 42) return confuse(x);
-  return x;
-}
+
+@AssumeDynamic() @NoInline()
+confuse(x) => x;
 
 foo(trace) {
   trace.add("foo");
   return "foo";
 }
 bar(trace) {
   trace.add("bar");
   return "bar";
 }
 
 main() {
   var f = confuse(foo);
   var b = confuse(bar);
 
   var trace = [];
   // Dart2js must keep the order of t1 and t2.
   var t1 = f(trace);
   var t2 = b(trace);
   var t3 = identical(t2, "foo");
   var t4 = trace.add(t1);
   trace.add(t3);
   trace.add(t3);
   Expect.listEquals(["foo", "bar", "foo", false, false], trace);
 }