Update analyzer SDK requirement

This CL causes the anaylzer to require an SDK version of at least
2.1.0-dev.5.0, which was the first version of the SDK in which
dart:core exported Future.

This change was already applied to the analyzer and analyzer-0.33
branches; this CL merely applies it to the master branch as well.  See
https://github.com/dart-lang/sdk/issues/35158.

Change-Id: I74ba071c5a2244a7ffa3d7ecadcfc9b2d7fbcf3f
Reviewed-on: https://dart-review.googlesource.com/c/85413
Commit-Queue: Paul Berry <paulberry@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Auto-Submit: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
diff --git a/pkg/analyzer/pubspec.yaml b/pkg/analyzer/pubspec.yaml
index ccae1e7..5675097 100644
--- a/pkg/analyzer/pubspec.yaml
+++ b/pkg/analyzer/pubspec.yaml
@@ -4,7 +4,7 @@
 description: Static analyzer for Dart.
 homepage: https://github.com/dart-lang/sdk/tree/master/pkg/analyzer
 environment:
-  sdk: '>=2.0.0-dev.48.0 <3.0.0'
+  sdk: '>=2.1.0-dev.5.0 <3.0.0'
 dependencies:
   args: '>=0.12.1 <2.0.0'
   charcode: ^1.1.0