Handle "as" in kernel with ir.AsExpression case.

tests/compiler/dart2js_extra/as_operator_test.dart

+// Copyright (c) 2016, 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";
+
+class A {}
+
+class B extends A {
+  int wobble() => 3;
+}
+class C extends A {
+  int foo() => 5;
+}
+
+main() {
+  A b = new B();
+  A c = new C();
+
+  Expect.equals(3, (b as B).wobble());
+  Expect.equals(5, (c as C).foo());

[sra1, 2016/11/29 03:39:47]

It is possible that these are optimized away completely.
You could make one of the 'as' expressions (b or c) have an input that is not
directly the HCreate with a known type.

if (confuse(false)) c = new A();

@NoInline() @AssumeDynamic()
confuse(x) => x;

If there is already a language test that does this, nevermind.

[Emily Fortuna, 2016/11/29 20:47:52]

found the language test in type_error_test. removing this test.

+
+  Expect.throws(() => (b as C));
+  Expect.throws(() => (b as C).foo());
+}