Fix package_test flakiness, init ensure terminal context entry is set (#10333)

packages/flutter_tools/lib/src/base/logger.dart

@@ -178,6 +178,7 @@ class BufferLogger extends Logger {
 
 class VerboseLogger extends Logger {
   VerboseLogger(this.parent) {
+    assert(terminal != null);
     stopwatch.start();
   }
 

packages/flutter_tools/test/src/context.dart

@@ -12,6 +12,7 @@ import 'package:flutter_tools/src/base/logger.dart';
 import 'package:flutter_tools/src/base/os.dart';
 import 'package:flutter_tools/src/base/platform.dart';
 import 'package:flutter_tools/src/base/port_scanner.dart';
+import 'package:flutter_tools/src/base/terminal.dart';
 import 'package:flutter_tools/src/cache.dart';
 import 'package:flutter_tools/src/devfs.dart';
 import 'package:flutter_tools/src/device.dart';
@@ -40,6 +41,7 @@ typedef void ContextInitializer(AppContext testContext);
 
 void _defaultInitializeContext(AppContext testContext) {
   testContext
+    ..putIfAbsent(AnsiTerminal, () => new AnsiTerminal())
     ..putIfAbsent(DeviceManager, () => new MockDeviceManager())
     ..putIfAbsent(DevFSConfig, () => new DevFSConfig())
     ..putIfAbsent(Doctor, () => new MockDoctor())