Fix all strong-mode warnings.

lib/src/handle_access_token_response.dart

 // Copyright (c) 2012, 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.
 
 import 'dart:convert';
 
+import 'package:collection/collection.dart';
 import 'package:http/http.dart' as http;
 import 'package:http_parser/http_parser.dart';
 
 import 'credentials.dart';
 import 'authorization_exception.dart';
 
 /// The amount of time to add as a "grace period" for credential expiration.
 ///
 /// This allows credential expiration checks to remain valid for a reasonable
 /// amount of time.
 const _expirationGrace = const Duration(seconds: 10);
 
 /// Handles a response from the authorization server that contains an access
 /// token.
 ///
 /// This response format is common across several different components of the
 /// OAuth2 flow.
 Credentials handleAccessTokenResponse(
     http.Response response,
     Uri tokenEndpoint,
     DateTime startTime,
     List<String> scopes) {
   if (response.statusCode != 200) _handleErrorResponse(response, tokenEndpoint);
 
   validate(condition, message) =>
       _validate(response, tokenEndpoint, condition, message);
 
-  var contentType = response.headers['content-type'];
-  if (contentType != null) contentType = new MediaType.parse(contentType);
+  var contentTypeString = response.headers['content-type'];
+  var contentType = contentTypeString == null
+      ? null
+      : new MediaType.parse(contentTypeString);
 
   // The spec requires a content-type of application/json, but some endpoints
   // (e.g. Dropbox) serve it as text/javascript instead.
   validate(contentType != null &&
       (contentType.mimeType == "application/json" ||
        contentType.mimeType == "text/javascript"),
       'content-type was "$contentType", expected "application/json"');
 
-  var parameters;
+  Map<String, dynamic> parameters;
   try {
-    parameters = JSON.decode(response.body);
+    var untypedParameters = JSON.decode(response.body);
+    validate(untypedParameters is Map,
+        'parameters must be a map, was "$parameters"');
+    parameters = DelegatingMap.typed(untypedParameters);
   } on FormatException catch (_) {
     validate(false, 'invalid JSON');
   }
 
   for (var requiredParameter in ['access_token', 'token_type']) {
     validate(parameters.containsKey(requiredParameter),
         'did not contain required parameter "$requiredParameter"');
     validate(parameters[requiredParameter] is String,
         'required parameter "$requiredParameter" was not a string, was '
         '"${parameters[requiredParameter]}"');
   }
 
   // TODO(nweiz): support the "mac" token type
   // (http://tools.ietf.org/html/draft-ietf-oauth-v2-http-mac-01)
   validate(parameters['token_type'].toLowerCase() == 'bearer',
       '"$tokenEndpoint": unknown token type "${parameters['token_type']}"');
 
   var expiresIn = parameters['expires_in'];
   validate(expiresIn == null || expiresIn is int,
       'parameter "expires_in" was not an int, was "$expiresIn"');
 
   for (var name in ['refresh_token', 'scope']) {
     var value = parameters[name];
     validate(value == null || value is String,
         'parameter "$name" was not a string, was "$value"');
   }
 
-  var scope = parameters['scope'];
+  var scope = parameters['scope'] as String;
   if (scope != null) scopes = scope.split(" ");
 
   var expiration = expiresIn == null ? null :
       startTime.add(new Duration(seconds: expiresIn) - _expirationGrace);
 
   return new Credentials(
       parameters['access_token'],
       refreshToken: parameters['refresh_token'],
       tokenEndpoint: tokenEndpoint,
       scopes: scopes,
@@ skipping 11 matching lines @@
   // off-spec.
   if (response.statusCode != 400 && response.statusCode != 401) {
     var reason = '';
     if (response.reasonPhrase != null && !response.reasonPhrase.isEmpty) {
       ' ${response.reasonPhrase}';
     }
     throw new FormatException('OAuth request for "$tokenEndpoint" failed '
         'with status ${response.statusCode}$reason.\n\n${response.body}');
   }
 
-  var contentType = response.headers['content-type'];
-  if (contentType != null) contentType = new MediaType.parse(contentType);
+  var contentTypeString = response.headers['content-type'];
+  var contentType = contentTypeString == null
+      ? null
+      : new MediaType.parse(contentTypeString);
+
   validate(contentType != null && contentType.mimeType == "application/json",
       'content-type was "$contentType", expected "application/json"');
 
   var parameters;
   try {
     parameters = JSON.decode(response.body);
   } on FormatException catch (_) {
     validate(false, 'invalid JSON');
   }
 
@@ skipping 17 matching lines @@
 
 void _validate(
     http.Response response,
     Uri tokenEndpoint,
     bool condition,
     String message) {
   if (condition) return;
   throw new FormatException('Invalid OAuth response for "$tokenEndpoint": '
       '$message.\n\n${response.body}');
 }