Re-enable implementation_imports lint rule.

Author: pq

packages/flutter_tools/flutter_analysis_options

@@ -36,7 +36,7 @@ linter:
     # - 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
+    - implementation_imports
     - library_names
     - library_prefixes
     - non_constant_identifier_names

packages/flutter_tools/lib/src/dart/analysis.dart

@@ -11,17 +11,17 @@ import 'package:analyzer/plugin/options.dart';
 import 'package:analyzer/source/analysis_options_provider.dart';
 import 'package:analyzer/source/embedder.dart';
 import 'package:analyzer/source/error_processor.dart';
-import 'package:analyzer/src/generated/engine.dart';
-import 'package:analyzer/src/generated/error.dart';
-import 'package:analyzer/src/generated/java_io.dart';
-import 'package:analyzer/src/generated/sdk_io.dart';
-import 'package:analyzer/src/generated/source.dart';
-import 'package:analyzer/src/generated/source_io.dart';
-import 'package:analyzer/src/task/options.dart';
+import 'package:analyzer/src/generated/engine.dart'; // ignore: implementation_imports
+import 'package:analyzer/src/generated/error.dart'; // ignore: implementation_imports
+import 'package:analyzer/src/generated/java_io.dart'; // ignore: implementation_imports
+import 'package:analyzer/src/generated/sdk_io.dart'; // ignore: implementation_imports
+import 'package:analyzer/src/generated/source.dart'; // ignore: implementation_imports
+import 'package:analyzer/src/generated/source_io.dart'; // ignore: implementation_imports
+import 'package:analyzer/src/task/options.dart'; // ignore: implementation_imports
 import 'package:cli_util/cli_util.dart' as cli_util;
-import 'package:linter/src/plugin/linter_plugin.dart';
+import 'package:linter/src/plugin/linter_plugin.dart'; // ignore: implementation_imports
 import 'package:package_config/packages.dart' show Packages;
-import 'package:package_config/src/packages_impl.dart' show MapPackages;
+import 'package:package_config/src/packages_impl.dart' show MapPackages; // ignore: implementation_imports
 import 'package:path/path.dart' as path;
 import 'package:plugin/manager.dart';
 import 'package:plugin/plugin.dart';

packages/flutter_tools/lib/src/usage.dart

@@ -4,7 +4,7 @@
 
 import 'dart:async';
 
-import 'package:usage/src/usage_impl_io.dart';
+import 'package:usage/src/usage_impl_io.dart'; // ignore: implementation_imports
 import 'package:usage/usage.dart';
 
 import 'base/context.dart';