[flutter_tools] check in script for generating per library unit coverage (#61996)

Allows generating a per-library coverage summary like https://gist.github.com/jonahwilliams/f298381c3fb9f472b2dfe54b82a20a88

packages/flutter_tools/test/integration.shard/unit_coverage_test.dart

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_tools/src/base/file_system.dart';
+import 'package:flutter_tools/src/base/io.dart';
+import 'package:flutter_tools/src/globals.dart' as globals;
+import 'package:process/process.dart';
+
+import '../src/common.dart';
+import 'test_utils.dart';
+
+void main() {
+  Directory tempDir;
+
+  setUp(() async {
+    tempDir = createResolvedTempDirectorySync('unit_coverage_test.');
+  });
+
+  tearDown(() async {
+    tryToDelete(tempDir);
+  });
+
+  test('Can parse and output summaries for code coverage', () async {
+    final File coverageFile = tempDir.childFile('info.lcov')
+      ..writeAsStringSync('''
+SF:lib/src/artifacts.dart
+DA:15,10
+DA:17,7
+DA:19,7
+DA:20,7
+DA:22,7
+DA:324,10
+DA:724,0
+DA:727,2
+DA:729,3
+DA:732,0
+DA:737,1
+LF:443
+LH:292
+end_of_record
+SF:lib/src/base/common.dart
+DA:13,7
+DA:14,7
+DA:22,7
+DA:27,3
+DA:28,6
+DA:40,7
+LF:6
+LH:6
+end_of_record
+''');
+
+    final String dartScript = globals.fs.path.join(getFlutterRoot(), 'bin', 'dart');
+    final String coverageScript = globals.fs.path.join(getFlutterRoot(), 'packages', 'flutter_tools', 'tool', 'unit_coverage.dart');
+    final ProcessResult result = await const LocalProcessManager().run(<String>[
+      dartScript,
+      coverageScript,
+      coverageFile.path,
+    ]);
+
+    // May contain other output if building flutter tool.
+    expect(result.stdout.toString().split('\n'), containsAll(<Matcher>[
+      contains('lib/src/artifacts.dart: 81.82%'),
+      contains('lib/src/base/common.dart: 100.00%'),
+    ]));
+  });
+}

packages/flutter_tools/tool/unit_coverage.dart

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:io';
+
+/// Produces a per-library coverage summary when fed an lcov file, sorted by
+/// increasing code coverage percentage.
+///
+/// Usage: `dart tool/unit_coverage lcov.info`
+void main(List<String> args) {
+  if (args.isEmpty || args.length > 1) {
+    print('Usage: dart tool/unit_coverage lcov.info');
+    return;
+  }
+  final List<String> lines = File(args.single).readAsLinesSync();
+  final List<Coverage> coverages = <Coverage>[];
+  Coverage currentCoverage;
+
+  for (final String line in lines) {
+    if (line.startsWith('SF:')) {
+      final String library = line.split('SF:')[1];
+      currentCoverage = Coverage()..library = library;
+      coverages.add(currentCoverage);
+    }
+    if (line.startsWith('DA')) {
+      currentCoverage.totalLines += 1;
+      if (!line.endsWith(',0')) {
+        currentCoverage.testedLines += 1;
+      }
+    }
+    if (line == 'end_of_record') {
+      currentCoverage = null;
+    }
+  }
+  coverages.sort((Coverage left, Coverage right) {
+    final double leftPercent = left.testedLines / left.totalLines;
+    final double rightPercent = right.testedLines / right.totalLines;
+    return leftPercent.compareTo(rightPercent);
+  });
+  for (final Coverage coverage in coverages) {
+    final String coveragePercent = (coverage.testedLines / coverage.totalLines * 100).toStringAsFixed(2);
+    print('${coverage.library}: $coveragePercent%');
+  }
+}
+
+class Coverage {
+  String library;
+  int totalLines = 0;
+  int testedLines = 0;
+}