dart2dart: Restore loops with conditions and updates in backend.

tests/language/rewrite_while_many_exits_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.
+
+import "package:expect/expect.dart";
+
+var baz_clicks = 0;
+
+baz() {
+    return ++baz_clicks;
+}
+
+var global = 0;
+
+increment_global() {
+    ++global;
+    return global <= 10;
+}
+
+foo(x, y) {
+    var n = 0;
+    while (true) {
+        baz();
+        if (n >= x) {
+            return n;
+        }
+        baz();
+        if (n >= y) {
+            return n;
+        }
+        n = n + 1;
+    }
+}
+
+bar() {
+    while (increment_global()) {
+        baz();
+    }
+    return baz();
+}
+
+main() {
+    Expect.equals(10, foo(10, 20));
+    Expect.equals(10, foo(20, 10));
+
+    baz_clicks = 0;
+    Expect.equals(11, bar());
+}