implement opassign, fix bugs in pre/postfix, introduce a let* helper

test/codegen/opassign.dart

+// Copyright (c) 2015, 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.
+
+get index {
+  print('called "index" getter');
+  return 0;
+}
+
+final _foo = new Foo();
+get foo {
+  print('called "foo" getter');
+  return _foo;
+}
+
+class Foo { int x = 100; }
+
+main() {
+  var f = { 0: 40 };
+  print('should only call "index" 2 times:');
+  ++f[index];
+  forcePostfix(f[index]++);
+
+  print('should only call "foo" 2 times:');
+  ++foo.x;
+  forcePostfix(foo.x++);
+
+  print('op assign test, should only call "index" twice:');
+  f[index] += f[index];
+}
+
+// Postfix generates as prefix if the value isn't used. This method prevents it.
+forcePostfix(x) {}