test: add test for template_cache_generator css rewriting

It use simple sed script to showcase integration is working.

Test runs only if sed is present on the running system.  The test
uses css with simple rules that are getting traslated for RTL locales.

Closes dart-archive#1052

test/io/template_cache_generator_spec.dart

@@ -70,6 +70,45 @@ void main() {
       });
     });
 
+    if (new File("/bin/sed").existsSync()) {
+      it('should correctly use simple sed rewritter to fake css mirroring', () {
+        var tmpDir = Directory.systemTemp.createTempSync();
+        Future flush;
+        try {
+          flush = generator.main([
+              '--out=${tmpDir.path}/generated.dart',
+              '--url-rewrites=/test/io/test_files,rewritten',
+              '--css-rewriter=test/io/test_files/rewritter.sh',
+              'test/io/test_files/cssUrls/main.dart',
+              'generated']);
+        } catch(_) {
+          tmpDir.deleteSync(recursive: true);
+          rethrow;
+        }
+        return flush.then((_) {
+          expect(new File('${tmpDir.path}/generated.dart').readAsStringSync(),
+'''// GENERATED, DO NOT EDIT!
+library generated;
 
+import 'package:angular/angular.dart';
+
+primeTemplateCache(TemplateCache tc) {
+tc.put("rewritten/cssUrls/four.css", new HttpResponse(200, r""".float-right {
+  float: right;
+}
+.right-margin {
+  margin-right: 20px;
+}
+
+"""));
+tc.put("rewritten/cssUrls/one.css", new HttpResponse(200, r"""body {}"""));
+tc.put("rewritten/cssUrls/three.css", new HttpResponse(200, r"""body {}"""));
+tc.put("rewritten/cssUrls/two.css", new HttpResponse(200, r"""body {}"""));
+}''');
+        }).whenComplete(() {
+          tmpDir.deleteSync(recursive: true);
+        });
+      });
+    }
   });
 }

test/io/test_files/cssUrls/four.css

@@ -0,0 +1,7 @@
+.float-left {
+  float: left;
+}
+.left-margin {
+  margin-left: 20px;
+}
+

test/io/test_files/rewritter.sh

@@ -0,0 +1,3 @@
+#!/bin/bash
+
+/bin/sed 's/left/right/g'