Regularize some naming to make BUILD.gn files easier to write.

sdk/lib/core/corelib_sources.gypi

Index: sdk/lib/core/corelib_sources.gypi
===================================================================
--- sdk/lib/core/corelib_sources.gypi	(revision 41495)
+++ sdk/lib/core/corelib_sources.gypi	(working copy)
@@ -1,43 +0,0 @@
-# Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
-# for details. All rights reserved. Use of this source code is governed by a
-# BSD-style license that can be found in the LICENSE file.
-
-{
-  'sources': [
-    'core.dart',
-    # The above file needs to be first as it lists the parts below.
-    'annotations.dart',
-    'bool.dart',
-    'comparable.dart',
-    'date_time.dart',
-    'double.dart',
-    'duration.dart',
-    'errors.dart',
-    'exceptions.dart',
-    'expando.dart',
-    'function.dart',
-    'identical.dart',
-    'int.dart',
-    'invocation.dart',
-    'iterable.dart',
-    'iterator.dart',
-    'list.dart',
-    'map.dart',
-    'null.dart',
-    'num.dart',
-    'object.dart',
-    'pattern.dart',
-    'print.dart',
-    'regexp.dart',
-    'set.dart',
-    'sink.dart',
-    'stacktrace.dart',
-    'stopwatch.dart',
-    'string.dart',
-    'string_buffer.dart',
-    'string_sink.dart',
-    'symbol.dart',
-    'type.dart',
-    'uri.dart',
-  ],
-}