enable lint prefer_foreach (#12674)

* enable lint prefer_foreach

* fix tests

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

diff --git a/packages/flutter_tools/lib/src/base/net.dart b/packages/flutter_tools/lib/src/base/net.dart
index 1d9adfb..52ef656 100644
--- a/packages/flutter_tools/lib/src/base/net.dart
+++ b/packages/flutter_tools/lib/src/base/net.dart
@@ -55,8 +55,7 @@
   printTrace('Received response from server, collecting bytes...');
   try {
     final BytesBuilder responseBody = new BytesBuilder(copy: false);
-    await for (List<int> chunk in response)
-      responseBody.add(chunk);
+    await response.forEach(responseBody.add);
     return responseBody.takeBytes();
   } on IOException catch (error) {
     printTrace('Download error: $error');

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

diff --git a/packages/flutter_tools/lib/src/base/utils.dart b/packages/flutter_tools/lib/src/base/utils.dart
index af7b8b8..2a4fee1 100644
--- a/packages/flutter_tools/lib/src/base/utils.dart
+++ b/packages/flutter_tools/lib/src/base/utils.dart
@@ -142,10 +142,8 @@
 
     _items = updatedSet;
 
-    for (T item in addedItems)
-      _addedController.add(item);
-    for (T item in removedItems)
-      _removedController.add(item);
+    addedItems.forEach(_addedController.add);
+    removedItems.forEach(_removedController.add);
   }
 
   /// Close the streams.