test that file property is being added to sourcemap (#986)

test/cli/sourcemap-file/_config.js

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+const assert = require( 'assert' );
+
+module.exports = {
+	description: 'populates file property of sourcemap (#986)',
+	command: 'rollup -i main.js -o _actual/bundle.js -m'
+};

test/cli/sourcemap-file/_expected/bundle.js

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+console.log( 42 );
+//# sourceMappingURL=bundle.js.map

test/cli/sourcemap-file/_expected/bundle.js.map

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+{"version":3,"file":"bundle.js","sources":["../main.js"],"sourcesContent":["console.log( 42 );\n"],"names":[],"mappings":"AAAA,OAAO,CAAC,GAAG,EAAE,EAAE,EAAE,CAAC"}

test/cli/sourcemap-file/main.js

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+console.log( 42 );