Give up tracing of a function if one of its closurizations escapes tracing.

tests/language/regress_21016_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.
+
+// Test that we give up on tracing a function if one of its closurizations
+// escapes tracing.
+
+class A {
+  var _boo = 22;
+  get boo {
+    return _boo;
+    return;
+  }
+}
+
+class B {
+  var _bar = 42;
+  get boo {
+    return _bar;
+    return;
+  }
+}
+
+class Holder {
+  tearMe(x) => x.boo;
+}
+
+var list = [];
+
+main () {
+  var holder = new Holder();
+  var hide = ((X) => X)(holder.tearMe);
+  hide(new A());
+  list.add(holder.tearMe);
+  var x = list[0];
+  x(new B());
+}