Source-map issue on windows

[3cp]

First, mozilla/source-map doesn't support \ as separator at all, because it's designed to be consumed in browser.

mozilla/source-map#91

1. the file and sourceRoot has to be normalized.
   

   component-compiler-utils/lib/parse.ts

   Lines 91 to 93 in 8de35e6

   	const map = new SourceMapGenerator({
   	file: filename,
   	sourceRoot

{
  file: filename.replace(/\\/g, '/'),
  sourceRoot: sourceRoot.replace(/\\/g, '/')
}

There is one more to be fixed in @vue/component-compiler.
https://github.com/vuejs/vue-component-compiler/blob/afa1cd440123e2e0c195908c1e15935273ac64a9/src/assembler.ts#L77

2. actually the default sourceRoot doesn't make much sense, because when serving the source-map to browser, process.cwd() is irrelevant.
   

   component-compiler-utils/lib/parse.ts

   Line 52 in 8de35e6

   sourceRoot = process.cwd(),

Change to

sourceRoot = '',

With the above two fixes, it can fix https://travis-ci.org/dumberjs/gulp-vue-file/builds/477568946
Update: I managed to fix the gulp-vue-file bug within gulp-vue-file by rewriting sourcemaps (normalize sources and file path). But the incoming sourcemaps should also be fixed in vue compiler.

Let me know what you think, I can make some PRs for this.

[haoqunjiang]

PRs are welcome :)