Use SPDX licences.

This is a followup to #3127.
At the time, SPDX licenses were pending approval by Google.

core/requires.js

@@ -1,18 +1,7 @@
 /**
  * @license
  * Copyright 2019 Google LLC
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *   http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
+ * SPDX-License-Identifier: Apache-2.0
  */
 
 /**
@@ -75,7 +64,7 @@ goog.require('Blockly.FieldNumber');
 goog.require('Blockly.FieldVariable');
 
 // If you'd like to include the date field in your build, you will also need to
-// include the closure library as a build dependency.  You can do so by running:
+// include the Closure library as a build dependency.  You can do so by running:
 //     gulp build-compressed --closure-library
 // Be sure to also include "google-closure-library" to your list of
 // devDependencies.