feat: adds support for 1:many source maps (bcoe#238)

Adds support for 1:many source-maps. These source maps are used by tools
like rollup or WebPack to bundle multiple JavaScript source files into one.

Refs: istanbuljs/v8-to-istanbul#102

Author: Benjamin E. Coe

package-lock.json

@@ -43,7 +43,7 @@
     "istanbul-reports": "^3.0.2",
     "rimraf": "^3.0.0",
     "test-exclude": "^6.0.0",
-    "v8-to-istanbul": "^4.1.2",
+    "v8-to-istanbul": "^5.0.0",
     "yargs": "^15.0.0",
     "yargs-parser": "^18.0.0"
   },

package.json

@@ -0,0 +1,12 @@
+const branch = require('./branch-2')
+
+if (false) {
+  console.info('unreachable')
+} else if (true) {
+  console.info('reachable')
+} else {
+  console.info('unreachable')
+}
+
+branch(true)
+branch(false)

test/fixtures/source-maps/branches/branch-1.js

@@ -0,0 +1,9 @@
+module.exports = function branch (a) {
+  if (a) {
+    console.info('a = true')
+  } else if (undefined) {
+    console.info('unreachable')
+  } else {
+    console.info('a = false')
+  }
+}

test/fixtures/source-maps/branches/branch-2.js

@@ -0,0 +1,5 @@
+const Foo = require('./class-2')
+
+const a = new Foo(0)
+const b = new Foo(33)
+a.methodA()

test/fixtures/source-maps/branches/branches.rollup.js

@@ -0,0 +1,23 @@
+module.exports = class Foo {
+  constructor (x=33) {
+    this.x = x ? x : 99
+    if (this.x) {
+      console.info('covered')
+    } else {
+      console.info('uncovered')
+    }
+    this.methodC()
+  }
+  methodA () {
+    console.info('covered')
+  }
+  methodB () {
+    console.info('uncovered')
+  }
+  methodC () {
+    console.info('covered')
+  }
+  methodD () {
+    console.info('uncovered')
+  }
+}