Make codegen_test more self contained

Addressed codegen_test `TODO(jmesserly): it'd be nice to do all cleanup
here …`.

pkg/dev_compiler/test/codegen_test.dart

@@ -81,13 +81,13 @@ main(arguments) {
   }
   var realSdk = getSdkDir(arguments).path;
 
-  // Validate that old output is gone before running.
-  // TODO(jmesserly): it'd be nice to do all cleanup here, including removing
-  // pub's 'packages' symlinks which mess up the diff. That way this test
-  // can be self contained instead of depending on a shell script.
-  if (new Directory(actualDir).existsSync()) {
-    throw 'Old compiler output should be cleaned up first. Use ./test/test.sh';
-  }
+  // Remove old output, and `packages` symlinks which mess up the diff.
+  var dir = new Directory(actualDir);
+  if (dir.existsSync()) dir.deleteSync(recursive: true);
+  var packagesDirs = new Directory(inputDir)
+      .listSync(recursive: true)
+      .where((d) => d is Directory && path.basename(d.path) == 'packages');
+  packagesDirs.forEach((d) => d.deleteSync());
 
   for (var filePath in paths) {
     var filename = path.basenameWithoutExtension(filePath);

pkg/dev_compiler/test/test.sh

@@ -35,11 +35,6 @@ cd $TEST_DIR/..
 
 ./tool/build_sdk.sh
 
-# Remove packages symlinks, and old codegen output
-find test/codegen -name packages -exec rm {} \;
-rm -r test/codegen/actual 2> /dev/null || true
-find test/dart_codegen -name packages -exec rm {} \;
-rm -r test/dart_codegen/actual 2> /dev/null || true
 dart -c test/all_tests.dart || fail
 
 # validate codegen_test output

pkg/dev_compiler/tool/travis.sh

@@ -4,7 +4,7 @@
 set -e
 
 function clean {
-  # This is a much more simple clean script, assuming git is available
+  # This is a much simpler clean script, assuming git is available
   pushd test
   git clean -fdx
   popd