Add script to combine coverage files (#10674)

* Add script to combine coverage files

* add script

* Add core coverage file as well

* Add json reporting

* Update README

* more

test/combineCoverage.js

@@ -0,0 +1,36 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the Source EULA. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+const iLibCoverage = require('istanbul-lib-coverage');
+const iLibReport = require('istanbul-lib-report');
+const iReports = require('istanbul-reports');
+const fs = require('fs');
+const path = require('path');
+const glob = require('glob');
+
+const coverageMap = iLibCoverage.createCoverageMap();
+const repoRoot = path.join(path.dirname(__filename), '..');
+
+// .build contain the core coverage files
+// extensions each may container their own coverage file
+const coverageFiles = glob.sync('{extensions,.build}/**/coverage-final.json',
+	{
+		cwd: repoRoot,
+		ignore: ['**/node_modules/**']
+	}
+);
+
+coverageFiles.forEach(file => {
+	console.log(`Merging coverage file ${path.join(repoRoot, file)}`);
+	coverageMap.merge(JSON.parse(fs.readFileSync(file)));
+});
+
+const context = iLibReport.createContext({
+	dir: path.join(path.dirname(__filename), 'coverage'),
+	coverageMap: coverageMap
+});
+
+const tree = context.getTree('flat');
+tree.visit(iReports.create('lcov'), context);