fix: Fix usage of excludeAfterRemap not to set the coverage always to 100

Do not use the `excludePath` callback. Remove the excluded sources at the end, after all coverage parts have been merged. This might be a problem in `v8-to-istanbul`, because `istanbul-lib-coverage` doesn't offer a method to filter the coverage data at the end.

Attempts to fix bcoe#462

Author: prantlf

lib/report.js

@@ -98,11 +98,14 @@ class Report {
       try {
         const sources = this._getSourceMap(v8ScriptCov)
         const path = resolve(this.resolve, v8ScriptCov.url)
-        const converter = v8toIstanbul(path, this.wrapperLength, sources, (path) => {
+        const converter = v8toIstanbul(path, this.wrapperLength, sources
+          // v8toIstanbul sets all coverage stats of the remaining files
+          // to 100, if some files are excluded here. See below.
+          /*, (path) => {
           if (this.excludeAfterRemap) {
             return !this._shouldInstrument(path)
           }
-        })
+        } */)
         await converter.load()
 
         if (resultCountPerPath.has(path)) {
@@ -131,6 +134,20 @@ class Report {
         map.merge(converter.toIstanbul())
       }
     }
+
+    // Workaround for too early applied excludeAfterRemap above -
+    // v8toIstanbul sets all coverage stats of the remaining files
+    // to 100, if some files are excluded by the excludePath callback.
+    if (this.excludeAfterRemap) {
+      const { data } = map
+      map.data = Object.keys(data).reduce((result, key) => {
+        if (this._shouldInstrument(key)) {
+          result[key] = data[key]
+        }
+        return result
+      }, {})
+    }
+
     this._allCoverageFiles = map
     return this._allCoverageFiles
   }

test/integration.js.snap

@@ -156,7 +156,7 @@ hey
 ---------------------------------------|---------|----------|---------|---------|-------------------
 File                                   | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
 ---------------------------------------|---------|----------|---------|---------|-------------------
-All files                              |    1.91 |       12 |    6.25 |    1.91 |                   
+All files                              |    1.89 |       12 |    6.25 |    1.89 |                   
  c8                                    |       0 |        0 |       0 |       0 |                   
   index.js                             |       0 |        0 |       0 |       0 | 1                 
  c8/bin                                |       0 |        0 |       0 |       0 |                   
@@ -168,7 +168,7 @@ All files                              |    1.91 |       12 |    6.25 |    1.91
  c8/lib                                |       0 |        0 |       0 |       0 |                   
   is-cjs-esm-bridge.js                 |       0 |        0 |       0 |       0 | 1-10              
   parse-args.js                        |       0 |        0 |       0 |       0 | 1-218             
-  report.js                            |       0 |        0 |       0 |       0 | 1-340             
+  report.js                            |       0 |        0 |       0 |       0 | 1-357             
   source-map-from-file.js              |       0 |        0 |       0 |       0 | 1-100             
  c8/lib/commands                       |       0 |        0 |       0 |       0 |                   
   check-coverage.js                    |       0 |        0 |       0 |       0 | 1-70