First batch of changes to make analyzer package strong mode error free

pkg/analyzer/lib/source/analysis_options_provider.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.source.analysis_options_provider;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/util/yaml.dart';
 import 'package:source_span/source_span.dart';
@@ skipping 50 matching lines @@
         }
         if (key is! String) {
           throw new OptionsFormatException(
               'Bad options file format (expected String scope key, '
               'got ${k.runtimeType})',
               k != null ? k.span : doc.span);
         }
         if (v != null && v is! YamlNode) {
           throw new OptionsFormatException(
               'Bad options file format (expected Node value, '
-                  'got ${v.runtimeType}: `${v.toString()}`)',
+              'got ${v.runtimeType}: `${v.toString()}`)',
               doc.span);
         }
         options[key] = v;
       });
     }
     return options;
   }
 
   /// Merge the given options contents where the values in [defaults] may be
   /// overridden by [overrides].
   ///
   /// Some notes about merge semantics:
   ///
   ///   * lists are merged (without duplicates).
   ///   * lists of scalar values can be promoted to simple maps when merged with
   ///     maps of strings to booleans (e.g., ['opt1', 'opt2'] becomes
   ///     {'opt1': true, 'opt2': true}.
   ///   * maps are merged recursively.
   ///   * if map values cannot be merged, the overriding value is taken.
   ///
   Map<String, YamlNode> merge(
           Map<String, YamlNode> defaults, Map<String, YamlNode> overrides) =>
-      new Merger().merge(defaults, overrides);
+      new Merger().merge(defaults, overrides) as Map<String, YamlNode>;
 
   /// Read the contents of [file] as a string.
   /// Returns null if file does not exist.
   String _readAnalysisOptionsFile(File file) {
     try {
       return file.readAsStringSync();
     } on FileSystemException {
       // File can't be read.
       return null;
     }
   }
 }
 
 /// Thrown on options format exceptions.
 class OptionsFormatException implements Exception {
   final String message;
   final SourceSpan span;
   OptionsFormatException(this.message, [this.span]);
 
   @override
   String toString() =>
       'OptionsFormatException: ${message?.toString()}, ${span?.toString()}';
 }