Depend on the new flx 0.0.5 and remove use of deprecated KeyPair.

packages/flutter_tools/lib/src/commands/build.dart

@@ -182,7 +182,7 @@ class BuildCommand extends FlutterCommand {
         archive.addFile(file);
     }
 
-    KeyPair keyPair = KeyPair.readFromPrivateKeySync(privateKeyPath);
+    AsymmetricKeyPair keyPair = keyPairFromPrivateKeyFileSync(privateKeyPath);
     Uint8List zipBytes = new Uint8List.fromList(new ZipEncoder().encode(archive));
     Bundle bundle = new Bundle.fromContent(
       path: outputPath,

packages/flutter_tools/pubspec.yaml

@@ -11,7 +11,7 @@ dependencies:
   analyzer: ">=0.26.1+17" # see note below
   archive: ^1.0.20
   args: ^0.13.0
-  flx: ">=0.0.1 <0.1.0"
+  flx: ">=0.0.5 <0.1.0"
   crypto: ^0.9.1
   mustache4dart: ^1.0.0
   path: ^1.3.0