Re-land r41544 "Update dart binaries used for running tests to version 1.7.2"

tools/testing/dart/http_server.dart

Index: tools/testing/dart/http_server.dart
diff --git a/tools/testing/dart/http_server.dart b/tools/testing/dart/http_server.dart
index a5e7b8eba1559afa31e963d5b7d42292adbeb68c..e45c27ddbbb069a3846b2dfe4bac602405df8ec9 100644
--- a/tools/testing/dart/http_server.dart
+++ b/tools/testing/dart/http_server.dart
@@ -56,7 +56,7 @@ class DispatchingServer {
 /// /root_build/X: This will serve the corresponding file from the build
 ///                directory (i.e. '$BuildDirectory/X').
 /// /FOO/packages/BAR: This will serve the corresponding file from the packages
-///                    directory (i.e. '$BuildDirectory/packages/BAR') or the 
+///                    directory (i.e. '$BuildDirectory/packages/BAR') or the
 ///                    passed-in package root
 /// /ws: This will upgrade the connection to a WebSocket connection and echo
 ///      all data back to the client.
@@ -144,7 +144,7 @@ class TestingServers {
     _buildDirectory = TestUtils.absolutePath(buildDirectory);
     _dartDirectory = dartDirectory == null ? TestUtils.dartDir
         : new Path(dartDirectory);
-    _packageRoot = packageRoot == null ? 
+    _packageRoot = packageRoot == null ?
       _buildDirectory.append('packages') :
       new Path(packageRoot);
   }
@@ -399,6 +399,8 @@ class TestingServers {
       response.headers.set('Content-Type', 'application/javascript');
     } else if (path.filename.endsWith('.dart')) {
       response.headers.set('Content-Type', 'application/dart');
+    } else if (path.filename.endsWith('.css')) {
+      response.headers.set('Content-Type', 'text/css');
     }
     response.headers.removeAll("X-Frame-Options");
     file.openRead().pipe(response).catchError((e) {