Build - reduce browser support to explicit browser versions

[kumavis]

• improves sourcemap validation tool
• fixes the browser target reference (previously was ignored)
• explicitly sets the browser target reference instead of indirect percentage based figure

            "chrome 55",
            "firefox 53",
            "opera 42",
            "edge 17",

( note: seems that opera reference is not supported at this version of preset-env )

  transform-class-properties
  check-es2015-constants {}
  transform-es2015-arrow-functions {}
  transform-es2015-block-scoped-functions {}
  transform-es2015-block-scoping {}
  transform-es2015-classes {}
  transform-es2015-computed-properties {}
  transform-es2015-destructuring {}
  transform-es2015-duplicate-keys {}
  transform-es2015-for-of {}
  transform-es2015-function-name {}
  transform-es2015-literals {}
  transform-es2015-object-super {}
  transform-es2015-parameters {}
  transform-es2015-shorthand-properties {}
  transform-es2015-spread {}
  transform-es2015-sticky-regex {}
  transform-es2015-template-literals {}
  transform-es2015-typeof-symbol {}
  transform-es2015-unicode-regex {}
  transform-regenerator {}
  transform-exponentiation-operator {}
  transform-async-to-generator {}
  syntax-trailing-function-commas {}

after:

  transform-class-properties
  transform-es2015-parameters
  transform-es2015-computed-properties
  transform-es2015-spread
  transform-es2015-destructuring {"edge":"17"}
  transform-es2015-function-name {"edge":"17"}
  syntax-trailing-function-commas {"chrome":"55"}

[kumavis]

test:unit fails without ending for some reason...

[whymarrh]

• explicitly sets the browser target reference instead of indirect percentage based figure

I'm not sure that's an improvement—using the percentage is one way of us reducing our bundle size automatically over time as browsers support more things out of the box. The >0.25% would allow us to drop old Firefox and Chrome versions, since they auto-update (i.e. they wouldn't include Firefox 53, which is added here explicitly^[1]).

We could rebase this and merge it without 2901bc1 and maybe split that into its own PR?

[danjm]

Closing for now. Feel free to reopen.