[cloud_firestore]bump up flutter version to 1.5 and use invokeMapMethod instead of invokeMethod (#1650)

diff --git a/packages/cloud_firestore/CHANGELOG.md b/packages/cloud_firestore/CHANGELOG.md
index 066d9ea..78265c1 100644
--- a/packages/cloud_firestore/CHANGELOG.md
+++ b/packages/cloud_firestore/CHANGELOG.md
@@ -1,3 +1,8 @@
+## 0.12.0+2
+
+* Bump the minimum Flutter version to 1.5.
+* Replace invokeMethod with invokeMapMethod wherever necessary.
+
 ## 0.12.0+1
 
 * Send user agent to Firebase.
diff --git a/packages/cloud_firestore/android/src/main/java/io/flutter/plugins/firebase/cloudfirestore/FlutterFirebaseAppRegistrar.java b/packages/cloud_firestore/android/src/main/java/io/flutter/plugins/firebase/cloudfirestore/FlutterFirebaseAppRegistrar.java
index d73d4b4..4c12000 100644
--- a/packages/cloud_firestore/android/src/main/java/io/flutter/plugins/firebase/cloudfirestore/FlutterFirebaseAppRegistrar.java
+++ b/packages/cloud_firestore/android/src/main/java/io/flutter/plugins/firebase/cloudfirestore/FlutterFirebaseAppRegistrar.java
@@ -8,7 +8,7 @@
 
 public class FlutterFirebaseAppRegistrar implements ComponentRegistrar {
   private static final String LIBRARY_NAME = "flutter-firebase_cloud_firestore";
-  private static final String LIBRARY_VERSION = "0.12.0+1";
+  private static final String LIBRARY_VERSION = "0.12.0+2";
 
   @Override
   public List<Component<?>> getComponents() {
diff --git a/packages/cloud_firestore/ios/Classes/CloudFirestorePlugin.m b/packages/cloud_firestore/ios/Classes/CloudFirestorePlugin.m
index a5a3bf0..c8e726f 100644
--- a/packages/cloud_firestore/ios/Classes/CloudFirestorePlugin.m
+++ b/packages/cloud_firestore/ios/Classes/CloudFirestorePlugin.m
@@ -7,7 +7,7 @@
 #import <Firebase/Firebase.h>
 
 #define LIBRARY_NAME @"flutter-firebase_cloud_firestore"
-#define LIBRARY_VERSION @"0.12.0+1"
+#define LIBRARY_VERSION @"0.12.0+2"
 
 static FlutterError *getFlutterError(NSError *error) {
   if (error == nil) return nil;
diff --git a/packages/cloud_firestore/lib/src/firestore.dart b/packages/cloud_firestore/lib/src/firestore.dart
index 94e6255..1b5eac3 100644
--- a/packages/cloud_firestore/lib/src/firestore.dart
+++ b/packages/cloud_firestore/lib/src/firestore.dart
@@ -112,14 +112,14 @@
         'Transaction timeout must be more than 0 milliseconds');
     final int transactionId = _transactionHandlerId++;
     _transactionHandlers[transactionId] = transactionHandler;
-    final Map<dynamic, dynamic> result = await channel
-        .invokeMethod<Map<dynamic, dynamic>>(
+    final Map<String, dynamic> result = await channel
+        .invokeMapMethod<String, dynamic>(
             'Firestore#runTransaction', <String, dynamic>{
       'app': app.name,
       'transactionId': transactionId,
       'transactionTimeout': timeout.inMilliseconds
     });
-    return result?.cast<String, dynamic>() ?? <String, dynamic>{};
+    return result ?? <String, dynamic>{};
   }
 
   @deprecated
diff --git a/packages/cloud_firestore/lib/src/transaction.dart b/packages/cloud_firestore/lib/src/transaction.dart
index bacc5a0..d42c89d 100644
--- a/packages/cloud_firestore/lib/src/transaction.dart
+++ b/packages/cloud_firestore/lib/src/transaction.dart
@@ -14,9 +14,8 @@
   Firestore _firestore;
 
   Future<DocumentSnapshot> get(DocumentReference documentReference) async {
-    final Map<dynamic, dynamic> result = await Firestore.channel
-        .invokeMethod<Map<dynamic, dynamic>>(
-            'Transaction#get', <String, dynamic>{
+    final Map<String, dynamic> result = await Firestore.channel
+        .invokeMapMethod<String, dynamic>('Transaction#get', <String, dynamic>{
       'app': _firestore.app.name,
       'transactionId': _transactionId,
       'path': documentReference.path,
diff --git a/packages/cloud_firestore/pubspec.yaml b/packages/cloud_firestore/pubspec.yaml
index 91d93ce..50dc5ae 100755
--- a/packages/cloud_firestore/pubspec.yaml
+++ b/packages/cloud_firestore/pubspec.yaml
@@ -3,7 +3,7 @@
   live synchronization and offline support on Android and iOS.
 author: Flutter Team <flutter-dev@googlegroups.com>
 homepage: https://github.com/flutter/plugins/tree/master/packages/cloud_firestore
-version: 0.12.0+1
+version: 0.12.0+2
 
 flutter:
   plugin:
@@ -27,4 +27,4 @@
 
 environment:
   sdk: ">=2.0.0-dev.28.0 <3.0.0"
-  flutter: ">=1.2.0 <2.0.0"
+  flutter: ">=1.5.0 <2.0.0"