Cope with mixed slashes on Windows when re-adding non-covered files

For whatever reason, the remapped coverage re-creates paths for source files with mixed slashes. This means that the reporter adds duplicate entries that differ only by the joining path separator, confusing the report writer, which fails to report the coverage on either entry.

src/reporter.js

@@ -102,8 +102,13 @@ function CoverageIstanbulReporter(baseReporterDecorator, logger, config) {
       .map;
 
     if (!coverageConfig.skipFilesWithNoCoverage) {
+      // On Windows, istanbul returns files with mixed forward/backslashes in them
+      const fixedFilePaths = {};
+      remappedCoverageMap.files().forEach(path => {
+        fixedFilePaths[util.fixPathSeparators(path)] = true;
+      });
       coverageMap.files().forEach(path => {
-        if (!(path in remappedCoverageMap)) {
+        if (!(util.fixPathSeparators(path) in fixedFilePaths)) {
           // Re-add empty coverage record
           remappedCoverageMap.addFileCoverage(path);
         }

src/util.js

@@ -90,6 +90,7 @@ function overrideThresholds(key, overrides, basePath) {
   return thresholds;
 }
 
+module.exports.fixPathSeparators = fixPathSeparators;
 module.exports.fixWebpackSourcePaths = fixWebpackSourcePaths;
 module.exports.fixWebpackFilePath = fixWebpackFilePath;
 module.exports.overrideThresholds = overrideThresholds;