Update links for China help (#25238)

We have a page on the Web page now.

bin/internal/update_dart_sdk.sh

@@ -53,18 +53,18 @@ if [ ! -f "$ENGINE_STAMP" ] || [ "$ENGINE_VERSION" != `cat "$ENGINE_STAMP"` ]; t
 
   curl --continue-at - --location --output "$DART_SDK_ZIP" "$DART_SDK_URL" 2>&1 || {
     echo
-    echo "Failed to retrieve the Dart SDK at $DART_SDK_URL"
-    echo "If you're located in China, please follow"
-    echo "https://github.com/flutter/flutter/wiki/Using-Flutter-in-China"
+    echo "Failed to retrieve the Dart SDK from: $DART_SDK_URL"
+    echo "If you're located in China, please see this page:"
+    echo "  https://flutter.io/community/china"
     echo
     rm -f -- "$DART_SDK_ZIP"
     exit 1
   }
   unzip -o -q "$DART_SDK_ZIP" -d "$FLUTTER_ROOT/bin/cache" || {
     echo
-    echo "It appears that the downloaded file is corrupt; please try the operation again later."
-    echo "If this problem persists, please report the problem at"
-    echo "https://github.com/flutter/flutter/issues/new?template=ACTIVATION.md"
+    echo "It appears that the downloaded file is corrupt; please try again."
+    echo "If this problem persists, please report the problem at:"
+    echo "  https://github.com/flutter/flutter/issues/new?template=ACTIVATION.md"
     echo
     rm -f -- "$DART_SDK_ZIP"
     exit 1

packages/flutter_tools/lib/src/cache.dart

@@ -221,9 +221,9 @@ class Cache {
     } on SocketException catch (e) {
       if (_hostsBlockedInChina.contains(e.address?.host)) {
         printError(
-          'Failed to retrieve Flutter tool depedencies: ${e.message}.\n'
-          "If you're in China, please follow "
-          'https://github.com/flutter/flutter/wiki/Using-Flutter-in-China',
+          'Failed to retrieve Flutter tool dependencies: ${e.message}.\n'
+          'If you\'re in China, please see this page: '
+          'https://flutter.io/community/china',
           emphasis: true,
         );
       }

packages/flutter_tools/test/cache_test.dart

@@ -98,7 +98,7 @@ void main() {
         verifyNever(artifact2.update());
         expect(
           testLogger.errorText,
-          contains('https://github.com/flutter/flutter/wiki/Using-Flutter-in-China'),
+          contains('https://flutter.io/community/china'),
         );
       }
     });