fix(preprocessor): Change paths in windows to use backslash

When istanbul creates a report combining json files from multiple
sources with different path separaters, all files are listed with the
full absolute path.

If path separators are consistent between json files, all files are
relative to a common base, and everything looks much nicer.

Closes #178

lib/preprocessor.js

@@ -76,6 +76,7 @@ function createCoveragePreprocessor (logger, helper, basePath, reporters, covera
   return function (content, file, done) {
     log.debug('Processing "%s".', file.originalPath)
 
+    var jsPath = require.resolve(file.originalPath)
     // default instrumenters
     var instrumenterLiteral = 'istanbul'
 
@@ -105,7 +106,7 @@ function createCoveragePreprocessor (logger, helper, basePath, reporters, covera
     options = extend(options, {codeGenerationOptions: codeGenerationOptions})
 
     var instrumenter = new InstrumenterConstructor(options)
-    instrumenter.instrument(content, file.originalPath, function (err, instrumentedCode) {
+    instrumenter.instrument(content, jsPath, function (err, instrumentedCode) {
       if (err) {
         log.error('%s\n  at %s', err.message, file.originalPath)
       }
@@ -120,7 +121,7 @@ function createCoveragePreprocessor (logger, helper, basePath, reporters, covera
       }
 
       // remember the actual immediate instrumented JS for given original path
-      sourceCache[file.originalPath] = content
+      sourceCache[jsPath] = content
 
       if (includeAllSources) {
         var coverageObjMatch = coverageObjRegex.exec(instrumentedCode)