Add more tests for boxed variables.

tests/compiler/dart2js/closure/data/mixed.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.
+
+// Test that free variables aren't mixed between capturing and non-capturing
+// closures.
+
+/*mutateInClosure:box=box0,boxed=[local]*/
+mutateInClosure() {
+  var /*boxed*/ local;
+  /*free=[box0,local]*/ () {
+    local = 42;
+  };
+  /**/ () {
+    // Use nothing.
+  };
+  return local;
+}
+
+/*mutateOutsideClosure:box=box0,boxed=[local]*/
+mutateOutsideClosure() {
+  var /*boxed*/ local;
+  /*free=[box0,local]*/ () {
+    print(local);
+  };
+  /**/ () {
+    // Use nothing.
+  };
+  local = 42;
+  return local;
+}
+
+/*mutateInOtherClosure:box=box0,boxed=[local]*/
+mutateInOtherClosure() {
+  var /*boxed*/ local;
+  /*free=[box0,local]*/ () {
+    print(local);
+  };
+  /*free=[box0,local]*/ () {
+    local = 42;
+  };
+  /**/ () {
+    // Use nothing.
+  };
+  return local;
+}
+
+/*mutateInNestedClosure:box=box0,boxed=[local]*/
+mutateInNestedClosure() {
+  var /*boxed*/ local;
+  /*free=[box0,local]*/ () {
+    print(local);
+    /*free=[box0,local]*/ () {
+      local = 42;
+    };
+    /**/ () {
+      // Use nothing.
+    };
+  };
+  /**/ () {
+    // Use nothing.
+  };
+  return local;
+}
+
+main() {
+  mutateInClosure();
+  mutateOutsideClosure();
+  mutateInOtherClosure();
+  mutateInNestedClosure();
+}