[ddc] Bump language version to 3.6

Preparation for the bump to 3.7 and reformatting all the source code
in pkg/dev_compiler. This makes it obvious that the next bump will
only contain changes needed for 3.7.

Change-Id: I997358aea4fcf3d899490611353df8b067bde929
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/401387
Reviewed-by: Mayank Patke <[email protected]>
Commit-Queue: Nicholas Shahan <[email protected]>

Author: nshahan

pkg/dev_compiler/pubspec.yaml

@@ -3,7 +3,7 @@ name: dev_compiler
 publish_to: none
 
 environment:
-  sdk: '>=3.0.0 <4.0.0'
+  sdk: '>=3.6.0 <4.0.0'
 
 # Use 'any' constraints here; we get our versions from the DEPS file.
 dependencies: