New `whitespace_around_ops` lint (#249).

test/rules/whitespace_around_ops.dart

+// Copyright (c) 2016, 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 w/ `dart test/util/solo_test.dart whitespace_around_ops`
+
+
+// Define our own int so we don't need `int` in our mock SDK.
+class MyInt {
+  MyInt operator -() => this;
+  MyInt operator ~() => this;
+  MyInt operator ~/(MyInt other) => this;
+  MyInt operator /(MyInt other) => this;
+}
+
+void main() {
+  MyInt f, g;
+  print(f ~/ g);
+  print(f~/ g); //LINT
+  print(f ~/g); //LINT
+  print(f~/g); //LINT
+  print(f/ ~g); //LINT
+  print(f /~g); //LINT
+  print(f / ~g); //OK
+  f =- g; //LINT
+  f = -g; //OK
+}