Add analysis options near the root of the repository (#3700)

This aligns the analysis options used by 'flutter analyze' with the
options used by Atom.

examples/stocks/.analysis_options

@@ -1,3 +0,0 @@
-analyzer:
-  exclude:
-    - 'lib/i18n/stock_messages_*.dart'

packages/.analysis_options

@@ -0,0 +1,55 @@
+# Specify analysis options.
+#
+# Note that until there is a default "all-in" lint rule-set we need
+# to opt-in to all desired lints (https://github.com/dart-lang/sdk/issues/25843).
+# For a list of lints, see: http://dart-lang.github.io/linter/lints/
+
+# This file is the .analysis_options file used by Flutter editors, such as
+# Atom. It is very similar to flutter_tools/flutter_analysis_options; the only
+# difference (currently) is the public_member_api_docs option, which triggers
+# too many messages to be used in editors.
+
+analyzer:
+  language:
+    enableStrictCallChecks: true
+    enableSuperMixins: true
+  strong-mode: true
+  errors:
+    # we allow overriding fields (if they use super, ideally...)
+    strong_mode_invalid_field_override: ignore
+    # we allow type narrowing
+    strong_mode_invalid_method_override: ignore
+    strong_mode_static_type_error: ignore
+    strong_mode_down_cast_composite: ignore
+    # we allow having TODOs in the code
+    todo: ignore
+linter:
+  rules:
+    - avoid_empty_else
+    - always_declare_return_types
+    - always_specify_types
+    - annotate_overrides
+    - avoid_as
+    - avoid_init_to_null
+    # - avoid_return_types_on_setters # https://github.com/dart-lang/linter/issues/202
+    - camel_case_types
+    # - constant_identifier_names # https://github.com/dart-lang/linter/issues/204 (and 203)
+    - empty_constructor_bodies
+    - hash_and_equals
+    # - implementation_imports # https://github.com/dart-lang/linter/issues/203
+    - library_names
+    - library_prefixes
+    - non_constant_identifier_names
+    # - one_member_abstracts # https://github.com/dart-lang/linter/issues/203
+    - package_api_docs
+    - package_names
+    - package_prefixed_library_names
+    - prefer_is_not_empty
+    - slash_for_doc_comments
+    - sort_constructors_first
+    - sort_unnamed_constructors_first
+    - super_goes_last
+    - type_annotate_public_apis # subset of always_specify_types
+    - type_init_formals
+    - unnecessary_brace_in_string_interp
+    - unnecessary_getters_setters