Enforce prefer_typing_uninitialized_variables lint.

R=brianwilkerson@google.com

Change-Id: Icfe4a24a6e69ab08f4743502ae88f94ab9cd8c8a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/153951
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer/analysis_options.yaml

@@ -18,5 +18,6 @@ linter:
   rules:
     - avoid_unused_constructor_parameters
     - empty_statements
+    - prefer_typing_uninitialized_variables
     - unnecessary_brace_in_string_interps
     - unnecessary_parenthesis

pkg/analyzer/lib/src/dart/analysis/driver.dart

@@ -2278,7 +2278,7 @@ class _DiscoverAvailableFilesTask {
 
 /// Information about an exception and its context.
 class _ExceptionState {
-  final exception;
+  final Object exception;
   final StackTrace stackTrace;
 
   /// The key under which the context of the exception was stored, or `null`

pkg/analyzer/lib/src/error/best_practices_verifier.dart

@@ -1526,9 +1526,9 @@ class _InvalidAccessVerifier {
     }
     AstNode grandparent = parent?.parent;
 
-    var element;
-    var name;
-    var node;
+    Element element;
+    String name;
+    AstNode node;
 
     if (grandparent is ConstructorName) {
       element = grandparent.staticElement;