[ddc] Bump pubspec SDK constraint to 3.0.0

Update abstract classes into proper mixins since that how they are
used.

Change-Id: I8ce43f873a2aacab48d859de633f531f091e58e6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/325143
Commit-Queue: Nicholas Shahan <nshahan@google.com>
Reviewed-by: Mayank Patke <fishythefish@google.com>

pkg/dev_compiler/lib/src/compiler/shared_compiler.dart

@@ -15,7 +15,7 @@ import '../js_ast/js_ast.dart' show js;
 ///
 /// This class should only implement functionality that depends purely on JS
 /// classes, rather than on Analyzer/Kernel types.
-abstract class SharedCompiler<Library extends Object, Class extends Object,
+mixin SharedCompiler<Library extends Object, Class extends Object,
     InterfaceType extends Object, FunctionNode extends Object> {
   /// When inside a `[]=` operator, this will be a non-null value that should be
   /// returned by any `return;` statement.

pkg/dev_compiler/lib/src/js_ast/nodes.dart

@@ -1943,7 +1943,7 @@ class Method extends Node implements Property {
 }
 
 /// Tag class for all interpolated positions.
-abstract class InterpolatedNode implements Node {
+mixin InterpolatedNode implements Node {
   dynamic get nameOrPosition;
 
   bool get isNamed => nameOrPosition is String;

pkg/dev_compiler/pubspec.yaml

@@ -3,7 +3,7 @@ name: dev_compiler
 publish_to: none
 
 environment:
-  sdk: '>=2.15.0 <3.0.0'
+  sdk: '>=3.0.0 <4.0.0'
 
 # Use 'any' constraints here; we get our versions from the DEPS file.
 dependencies: