Remove saving/restoring of the context at function entry.

tests/language/vm/closure_memory_retention_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.
+// VMOptions=--old_gen_heap_size=50
+
+// Test that non-capturing closures don't retain unnecessary memory.
+// It tests that the context of `f` allocated within `bar` not leaking and does
+// not become the context of empty non-capturing closure allocated inside `foo`.
+// If failing it crashes with an OOM error.
+
+import "package:expect/expect.dart";
+
+foo() {
+  return () { };
+}
+
+bar(a, b) {
+  f() => [a, b];
+  return foo();
+}
+
+main() {
+  var closure = null;
+  for (var i = 0; i < 100; i++) {
+    closure = bar(closure, new List(1024 * 1024));
+  }
+  Expect.isTrue(closure is Function);
+}