Reland "Suppress canonicalization of Unbox() instruction that can deoptimize."

tests/language/vm/canonicalization_preserves_deopt_test.dart

Index: tests/language/vm/canonicalization_preserves_deopt_test.dart
diff --git a/tests/language/vm/regress_21245_test.dart b/tests/language/vm/canonicalization_preserves_deopt_test.dart
similarity index 57%
copy from tests/language/vm/regress_21245_test.dart
copy to tests/language/vm/canonicalization_preserves_deopt_test.dart
index 8fa7bb086c4d2ffafd8e061c246373f04f419b61..b36e0eab71588150e5f4e8c284297016633b3513 100644
--- a/tests/language/vm/regress_21245_test.dart
+++ b/tests/language/vm/canonicalization_preserves_deopt_test.dart
@@ -3,21 +3,18 @@
 // BSD-style license that can be found in the LICENSE file.
 // VMOptions=--optimization_counter_threshold=10 --no-use-osr
 
-test(a) {
-  var e;
-  for (var i = 0; i < a.length; i++) {
-    e = a[i];
-    for (var j = 0; j < i; j++) {
-      e = a[j];
-    }
-  }
-  return e;
+import "package:expect/expect.dart";
+
+class X {
+  operator * (other) => "NaNNaNNaNNaNBatman";
 }
 
+foo(x) => (x * 1.0) is double;
+
 main() {
-  var a = [0, 1, 2, 3, 4, 5];
-  for (var i = 0; i < 20; i++) {
-    test(a);
+  for (var i = 0; i < 100; i++) {
+    Expect.isTrue(foo(1.0));
   }
+  Expect.isFalse(foo(new X()));
 }