Avoid completing multiplet times for one error (#454)

Also:
* updated SDK constraint to stable
* Use latest Dart action install
* Test on oldest supported SDK

.github/workflows/dart.yml

@@ -16,12 +16,12 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        sdk: [dev, beta]
+        sdk: [dev, 2.12.0]
     steps:
       - uses: actions/checkout@v2
-      - uses: cedx/setup-dart@v2
+      - uses: dart-lang/setup-dart@v1.0
         with:
-          release-channel: ${{ matrix.sdk }}
+          sdk: ${{ matrix.sdk }}
       - name: Report version
         run: dart --version
       - name: Install dependencies
@@ -50,7 +50,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        sdk: [dev, beta]
+        sdk: [dev, 2.12.0]
         platform: [vm, chrome]
         exclude:
           # We only run Chrome tests on Linux. No need to run them
@@ -61,9 +61,9 @@ jobs:
             platform: chrome
     steps:
       - uses: actions/checkout@v2
-      - uses: cedx/setup-dart@v2
+      - uses: dart-lang/setup-dart@v1.0
         with:
-          release-channel: ${{ matrix.sdk }}
+          sdk: ${{ matrix.sdk }}
       - name: Report version
         run: dart --version
       - name: Install envoy

CHANGELOG.md

@@ -1,3 +1,5 @@
+## 3.0.1-dev
+
 ## 3.0.0
 
 * Migrate library and tests to null safety.

interop/lib/src/client.dart

@@ -1108,12 +1108,14 @@ class Tester {
       }
       call.cancel();
     }, onError: (e) {
-      if (e is! GrpcError) completer.completeError('Unexpected error: $e.');
-      if (e.code != StatusCode.cancelled) {
+      if (e is! GrpcError) {
+        completer.completeError('Unexpected error: $e.');
+      } else if (e.code != StatusCode.cancelled) {
         completer
             .completeError('Unexpected status code ${e.code}: ${e.message}.');
+      } else {
+        completer.complete(true);
       }
-      completer.complete(true);
     }, onDone: () {
       if (!completer.isCompleted) completer.completeError('Expected error.');
     });

pubspec.yaml

@@ -1,12 +1,12 @@
 name: grpc
 description: Dart implementation of gRPC, a high performance, open-source universal RPC framework.
 
-version: 3.0.0
+version: 3.0.1-dev
 
 repository: https://github.com/grpc/grpc-dart
 
 environment:
-  sdk: '>=2.12.0-0 <3.0.0'
+  sdk: '>=2.12.0 <3.0.0'
 
 dependencies:
   archive: ^3.0.0
@@ -22,7 +22,7 @@ dependencies:
 dev_dependencies:
   build_runner: ^1.11.0
   build_test: ^1.3.4
-  mockito: ^5.0.0-nullsafety.6
+  mockito: ^5.0.0
   path: ^1.8.0
   test: ^1.16.0
   stream_channel: ^2.1.0