Add type inference for assert statements.

pkg/front_end/testcases/inference/assert.dart

+// Copyright (c) 2017, 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.
+
+/*@testedFeatures=inference*/
+library test;
+
+T f<T>() => null;
+
+void test() {
+  assert(/*@typeArgs=dynamic*/ f());
+  assert(/*@typeArgs=dynamic*/ f(), /*@typeArgs=dynamic*/ f());
+}
+
+main() {}