test_runner: account for newline in source maps

lib/internal/test_runner/coverage.js

@@ -340,8 +340,8 @@ class TestCoverage {
       const { data, lineLengths } = sourceMapCache[url];
       let offset = 0;
       const executedLines = ArrayPrototypeMap(lineLengths, (length, i) => {
-        const coverageLine = new CoverageLine(i + 1, offset, null, length);
-        offset += length;
+        const coverageLine = new CoverageLine(i + 1, offset, null, length + 1);
+        offset += length + 1;
         return coverageLine;
       });
       if (data.sourcesContent != null) {

test/fixtures/test-runner/source-map-line-lengths/index.ts

@@ -0,0 +1,76 @@
+1;
+1;
+1;
+1;
+1;
+1;
+1;
+1;
+1;
+1;
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+1;
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+1;
+1;
+1;
+1;
+1;
+1;
+1;
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+1;
+1;
+1;
+1;
+1;
+1;
+1;
+function a() {
+  console.log(1);
+}
+a();

test/parallel/test-runner-coverage.js

@@ -428,3 +428,16 @@ test('coverage with included and excluded files', skipIfNoInspector, () => {
   assert.strictEqual(result.status, 0);
   assert(!findCoverageFileForPid(result.pid));
 });
+
+test('properly accounts for line endings in source maps', skipIfNoInspector, () => {
+  const fixture = fixtures.path('test-runner', 'source-map-line-lengths', 'index.js');
+  const args = [
+    '--test', '--experimental-test-coverage', '--test-reporter', 'tap',
+    fixture,
+  ];
+  const result = spawnSync(process.execPath, args);
+  const report = 'index.ts | 100.00 |   100.00 |  100.00 |';
+  assert.strictEqual(result.stderr.toString(), '');
+  assert(result.stdout.toString().includes(report));
+  assert.strictEqual(result.status, 0);
+});