Coverage: fix installHook API (#10747)

- remove unused collector parameter
- clarify a comment
- inline _currentPackageTestDir

packages/flutter_tools/lib/src/commands/test.dart

diff --git a/packages/flutter_tools/lib/src/commands/test.dart b/packages/flutter_tools/lib/src/commands/test.dart
index 6171abc..a26ea29 100644
--- a/packages/flutter_tools/lib/src/commands/test.dart
+++ b/packages/flutter_tools/lib/src/commands/test.dart
@@ -72,12 +72,6 @@
   @override
   String get description => 'Run Flutter unit tests for the current project.';
 
-  Directory get _currentPackageTestDir {
-    // We don't scan the entire package, only the test/ subdirectory, so that
-    // files with names like like "hit_test.dart" don't get run.
-    return fs.directory('test');
-  }
-
   Future<bool> _collectCoverageData(CoverageCollector collector, { bool mergeCoverageData: false }) async {
     final Status status = logger.startProgress('Collecting coverage information...');
     final String coverageData = await collector.finalizeCoverage(
@@ -159,7 +153,9 @@
 
     Directory workDir;
     if (files.isEmpty) {
-      workDir = _currentPackageTestDir;
+      // We don't scan the entire package, only the test/ subdirectory, so that
+      // files with names like like "hit_test.dart" don't get run.
+      workDir = fs.directory('test');
       if (!workDir.existsSync())
         throwToolExit('Test directory "${workDir.path}" not found.');
       files = _findTests(workDir);

packages/flutter_tools/lib/src/test/flutter_platform.dart

diff --git a/packages/flutter_tools/lib/src/test/flutter_platform.dart b/packages/flutter_tools/lib/src/test/flutter_platform.dart
index 8872f79..e3999f3 100644
--- a/packages/flutter_tools/lib/src/test/flutter_platform.dart
+++ b/packages/flutter_tools/lib/src/test/flutter_platform.dart
@@ -18,7 +18,6 @@
 import '../base/process_manager.dart';
 import '../dart/package_map.dart';
 import '../globals.dart';
-import 'coverage_collector.dart';
 import 'watcher.dart';
 
 /// The timeout we give the test process to connect to the test harness
@@ -53,7 +52,6 @@
 /// main()`), you can set an observatory port and a diagnostic port explicitly.
 void installHook({
   @required String shellPath,
-  CoverageCollector collector,
   TestWatcher watcher,
   bool enableObservatory: false,
   bool startPaused: false,

packages/flutter_tools/lib/src/test/runner.dart

diff --git a/packages/flutter_tools/lib/src/test/runner.dart b/packages/flutter_tools/lib/src/test/runner.dart
index 7bc9572..80f37e1 100644
--- a/packages/flutter_tools/lib/src/test/runner.dart
+++ b/packages/flutter_tools/lib/src/test/runner.dart
@@ -55,8 +55,8 @@
     serverType: serverType,
   );
 
-  // Set the package path used for child processes.
-  // TODO(skybrian): why is this global? Move to installHook?
+  // Make the global packages path absolute.
+  // (Makes sure it still works after we change the current directory.)
   PackageMap.globalPackagesPath =
       fs.path.normalize(fs.path.absolute(PackageMap.globalPackagesPath));