Revert "Revert "Use runTests in fuchsia tester. (#19178)" (#19321)" (#19327)

This reverts commit e22f9974.

Adding the exit(exitCode) fixes the hang.
Tested by running `fx run-host-tests` with a topaz checkout.
Verified the hang before, no hang after.

packages/flutter_tools/bin/fuchsia_tester.dart

@@ -11,12 +11,12 @@ import 'package:flutter_tools/src/base/io.dart';
 import 'package:flutter_tools/src/cache.dart';
 import 'package:flutter_tools/src/context_runner.dart';
 import 'package:flutter_tools/src/dart/package_map.dart';
+import 'package:flutter_tools/src/artifacts.dart';
 import 'package:flutter_tools/src/disabled_usage.dart';
 import 'package:flutter_tools/src/globals.dart';
-import 'package:flutter_tools/src/test/flutter_platform.dart' as loader;
+import 'package:flutter_tools/src/test/coverage_collector.dart';
+import 'package:flutter_tools/src/test/runner.dart';
 import 'package:flutter_tools/src/usage.dart';
-import 'package:test/src/executable.dart'
-    as test; // ignore: implementation_imports
 
 // Note: this was largely inspired by lib/src/commands/test.dart.
 
@@ -28,9 +28,11 @@ const List<String> _kRequiredOptions = const <String>[
   _kOptionShell,
   _kOptionTestDirectory,
 ];
+const String _kOptionCoverage = 'coverage';
+const String _kOptionCoveragePath = 'coverage-path';
 
-Future<Null> main(List<String> args) {
-  return runInContext<Null>(() => run(args), overrides: <Type, dynamic>{
+void main(List<String> args) {
+  runInContext<Null>(() => run(args), overrides: <Type, dynamic>{
     Usage: new DisabledUsage(),
   });
 }
@@ -47,12 +49,20 @@ Future<Null> run(List<String> args) async {
   final ArgParser parser = new ArgParser()
     ..addOption(_kOptionPackages, help: 'The .packages file')
     ..addOption(_kOptionShell, help: 'The Flutter shell binary')
-    ..addOption(_kOptionTestDirectory, help: 'Directory containing the tests');
+    ..addOption(_kOptionTestDirectory, help: 'Directory containing the tests')
+    ..addFlag(_kOptionCoverage,
+      defaultsTo: false,
+      negatable: false,
+      help: 'Whether to collect coverage information.',
+    )
+    ..addOption(_kOptionCoveragePath,
+        defaultsTo: 'coverage/lcov.info',
+        help: 'Where to store coverage information (if coverage is enabled).',
+    );
   final ArgResults argResults = parser.parse(args);
   if (_kRequiredOptions
       .any((String option) => !argResults.options.contains(option))) {
-    printError('Missing option! All options must be specified.');
-    exit(1);
+    throwToolExit('Missing option! All options must be specified.');
   }
   final Directory tempDirectory =
       fs.systemTempDirectory.createTempSync('fuchsia_tester');
@@ -70,17 +80,39 @@ Future<Null> run(List<String> args) async {
     if (!fs.isFileSync(shellPath)) {
       throwToolExit('Cannot find Flutter shell at $shellPath');
     }
-    loader.installHook(
-      shellPath: shellPath,
-    );
+    // Put the tester shell where runTests expects it.
+    // TODO(tvolkert,garymm): Switch to a Fuchsia-specific Artifacts impl.
+    final Link testerDestLink =
+        fs.link(artifacts.getArtifactPath(Artifact.flutterTester));
+    testerDestLink.parent.createSync(recursive: true);
+    testerDestLink.createSync(shellPath);
 
     PackageMap.globalPackagesPath =
         fs.path.normalize(fs.path.absolute(argResults[_kOptionPackages]));
-    fs.currentDirectory = testDirectory;
 
-    await test.main(testArgs);
-    exit(exitCode);
+    CoverageCollector collector;
+    if (argResults['coverage']) {
+      collector = new CoverageCollector();
+    }
+
+    exitCode = await runTests(
+      tests,
+      workDir: testDirectory,
+      watcher: collector,
+      enableObservatory: collector != null,
+    );
+
+    if (collector != null) {
+      // collector expects currentDirectory to be the root of the dart
+      // package (i.e. contains lib/ and test/ sub-dirs).
+      fs.currentDirectory = testDirectory.parent;
+      if (!await
+          collector.collectCoverageData(argResults[_kOptionCoveragePath]))
+        throwToolExit('Failed to collect coverage data');
+    }
   } finally {
     tempDirectory.deleteSync(recursive: true);
   }
+  // Not sure why this is needed, but main() doesn't seem to exit on its own.
+  exit(exitCode);
 }