fix(babel-transformer): respect top of the file comments/pragma (#2783)

Make sure top-level file comments are preserved, like `// @flow`. 

Co-authored-by: Nico Jansen <jansennico@gmail.com>

e2e/package.json

@@ -8,6 +8,7 @@
     "@babel/plugin-proposal-class-properties": "~7.8.3",
     "@babel/plugin-proposal-pipeline-operator": "~7.8.3",
     "@babel/preset-env": "~7.8.3",
+    "@babel/preset-flow": "7.12.13",
     "@types/node": "^10.12.18",
     "@types/semver": "~6.2.0",
     "ajv": "~7.0.2",

e2e/test/flow-test-project/.babelrc

@@ -0,0 +1,16 @@
+{
+    "presets": [
+        "@babel/preset-flow",
+        [
+            "@babel/preset-env",
+            {
+                "targets": {
+                    "edge": "16",
+                    "firefox": "57",
+                    "chrome": "62",
+                    "safari": "11"
+                }
+            }
+        ]
+    ]
+}

e2e/test/flow-test-project/package.json

@@ -0,0 +1,11 @@
+{
+  "name": "flow-jest-project",
+  "version": "0.0.0",
+  "private": true,
+  "description": "A module to test a flow project, see https://flow.org/",
+  "scripts": {
+    "test:unit": "jest",
+    "test": "stryker run",
+    "posttest": "mocha --no-config --require ../../tasks/ts-node-register.js verify/*.ts"
+  }
+}

e2e/test/flow-test-project/src/square.js

@@ -0,0 +1,6 @@
+// @flow
+export function square(n: number): number {
+  const result: Array<?number> = [];
+  const cat = new Set<string>();
+  return n * n;
+}

e2e/test/flow-test-project/src/square.spec.js

@@ -0,0 +1,8 @@
+import { square } from './square';
+import { expect } from 'chai';
+
+describe('square', () => {
+  it('should provide 4 when given 2', () => {
+    expect(square(2)).eq(4);
+  });
+});

e2e/test/flow-test-project/stryker.conf.json

@@ -0,0 +1,13 @@
+{
+  "$schema": "../../node_modules/@stryker-mutator/core/schema/stryker-schema.json",
+  "reporters": [
+    "clear-text",
+    "event-recorder"
+  ],
+  "concurrency": 2,
+  "commandRunner": {
+    "command": "npm run test:unit"
+  },
+  "symlinkNodeModules": false,
+  "fileLogLevel": "info"
+}

e2e/test/flow-test-project/verify/.babelrc

@@ -0,0 +1,16 @@
+{
+    "presets": [
+        "@babel/preset-flow",
+        [
+            "@babel/preset-env",
+            {
+                "targets": {
+                    "edge": "16",
+                    "firefox": "57",
+                    "chrome": "62",
+                    "safari": "11"
+                }
+            }
+        ]
+    ]
+}

e2e/test/flow-test-project/verify/verify.ts

@@ -0,0 +1,19 @@
+import { expectMetrics } from '../../../helpers';
+
+describe('After running stryker on jest-react project', () => {
+  it('should report expected scores', async () => {
+    await expectMetrics({
+      killed: 2,
+      ignored: 0,
+      survived: 1,
+      mutationScore: 66.67,
+    });
+    /*
+    -----------|---------|----------|-----------|------------|----------|---------|
+    File       | % score | # killed | # timeout | # survived | # no cov | # error |
+    -----------|---------|----------|-----------|------------|----------|---------|
+    All files  |   66.67 |        2 |         0 |          1 |        0 |       0 |
+     square.js |   66.67 |        2 |         0 |          1 |        0 |       0 |
+    -----------|---------|----------|-----------|------------|----------|---------|*/
+  });
+});

packages/instrumenter/src/transformers/babel-transformer.ts

@@ -40,6 +40,17 @@ export const transformBabel: AstTransformer<AstFormat.JS | AstFormat.TS> = (
     },
   });
   if (mutantCollector.hasPlacedMutants(originFileName)) {
-    root.program.body.unshift(...instrumentationBabelHeader);
+    // Be sure to leave comments like `// @flow` in.
+    let header = instrumentationBabelHeader;
+    if (Array.isArray(root.program.body[0]?.leadingComments)) {
+      header = [
+        {
+          ...instrumentationBabelHeader[0],
+          leadingComments: root.program.body[0]?.leadingComments,
+        },
+        ...instrumentationBabelHeader.slice(1),
+      ];
+    }
+    root.program.body.unshift(...header);
   }
 };

packages/instrumenter/src/util/syntax-helpers.ts

@@ -2,6 +2,7 @@ import { INSTRUMENTER_CONSTANTS as ID } from '@stryker-mutator/api/core';
 import { types, NodePath } from '@babel/core';
 import traverse from '@babel/traverse';
 import { parse } from '@babel/parser';
+import { deepFreeze } from '@stryker-mutator/util';
 
 import { Mutant } from '../mutant';
 
@@ -14,7 +15,8 @@ const IS_MUTANT_ACTIVE_HELPER = 'stryMutAct_9fa48';
 /**
  * Returns syntax for the header if JS/TS files
  */
-export const instrumentationBabelHeader = parse(`function ${STRYKER_NAMESPACE_HELPER}(){
+export const instrumentationBabelHeader = deepFreeze(
+  parse(`function ${STRYKER_NAMESPACE_HELPER}(){
   var g = new Function("return this")();
   var ns = g.${ID.NAMESPACE} || (g.${ID.NAMESPACE} = {});
   if (ns.${ID.ACTIVE_MUTANT} === undefined && g.process && g.process.env && g.process.env.${ID.ACTIVE_MUTANT_ENV_VARIABLE}) {
@@ -51,7 +53,8 @@ function ${IS_MUTANT_ACTIVE_HELPER}(id) {
   }
   ${IS_MUTANT_ACTIVE_HELPER} = isActive;
   return isActive(id);
-}`).program.body;
+}`).program.body
+) as readonly types.Statement[]; // cast here, otherwise the thing gets unwieldy to handle
 
 /**
  * returns syntax for `global.activeMutant === $mutantId`