Bump DEPS to linter 0.1.31.

Linter `0.1.31`:

* New `prefer_foreach` lint.
* New `use_string_buffers` rule.
* New `unnecessary_overrides` rule.
* New `join_return_with_assignment_when_possible` rule.
* New `use_to_and_as_if_applicable` rule.
* New `avoid_setters_without_getters` rule.
* New `always_put_control_body_on_new_line` rule.
* New `avoid_positional_boolean_parameters` rule.
* New `always_require_non_null_named_parameters` rule.
* New `prefer_conditional_assignment` rule.
* New `prefer_conditional_assignment` rule.
* New `avoid_types_on_closure_parameters` rule.
* New `always_put_control_body_on_new_line` rule.
* New `use_setters_to_change_properties` rule.
* New `avoid_returning_this` rule.
* New `avoid_annotating_with_dynamic_when_not_required` rule.
* New `prefer_constructors_over_static_methods` rule.
* New `avoid_returning_null` rule.
* New `avoid_classes_with_only_static_members` rule.
* New `avoid_null_checks_in_equality_operators` rule.
* New `avoid_catches_without_on_clauses` rule.
* New `avoid_catching_errors` rule.
* New `use_rethrow_when_possible` rule.
* Many lint fixes (notably `prefer_final_fields`, `unnecessary_lambdas`, `await_only_futures`, `cascade_invocations`, `avoid_types_on_closure_parameters`, and `overridden_fields`).
* Significant performance improvements for `prefer_interpolation_to_compose_strings`.
* New `unnecessary_this` rule.
* New `prefer_initiliazling_formals` rule.

BUG=
R=brianwilkerson@google.com

Review-Url: https://codereview.chromium.org/2865163002 .

DEPS

@@ -77,7 +77,7 @@ vars = {
   "isolate_tag": "@1.0.0",
   "jinja2_rev": "@2222b31554f03e62600cd7e383376a7c187967a1",
   "json_rpc_2_tag": "@2.0.4",
-  "linter_tag": "@0.1.30",
+  "linter_tag": "@0.1.31",
   "logging_tag": "@0.11.3+1",
   "markdown_tag": "@0.11.2",
   "matcher_tag": "@0.12.0+2",