Update tools to support Node 10

[usergenic]

.travis.yml

• Run tests on Node 10 instead of 9. (10.5+ supported; 10.4.x and under have a broken fs.chmod() that doesn't play nice with the way we use Gulp.)

polymer-build

• generateServiceWorker relies on Function.prototype.toString() to insert function source code into the template to produce service-worker.js. In Node 10, anonymous functions are emitted without the space between function keyword and the parentheses, whereas previously there was a space. Now the generator replaces /bfunction\(/ with function ( explicitly so that behavior is consistent (and also integration tests based on static fixtures work for both versions).
• Converted new Buffer( calls to Buffer.from( as constructor deprecated in Node 10.

polymer-cli

• /test/fixtures/build-with-preset/expected/es5-bundled/service-worker.js test fixture updated so that all anonymous functions use the same spacing around function keyword: function () { }

polyserve

• Converted new Buffer( calls to Buffer.from( as constructor deprecated in Node 10.

[aomarks]

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