Reduce strong mode errors and warnings

pkg/front_end/lib/src/fasta/source/stack_listener.dart

Index: pkg/front_end/lib/src/fasta/source/stack_listener.dart
diff --git a/pkg/front_end/lib/src/fasta/source/stack_listener.dart b/pkg/front_end/lib/src/fasta/source/stack_listener.dart
index 1438a2e19e66e2ec12d5ecef0647ab9c2aac4ab6..cba4a3eab3716f75be43c0731c6d244f222a69f5 100644
--- a/pkg/front_end/lib/src/fasta/source/stack_listener.dart
+++ b/pkg/front_end/lib/src/fasta/source/stack_listener.dart
@@ -55,7 +55,7 @@ abstract class StackListener extends Listener {
 
   // TODO(ahe): This doesn't belong here. Only implemented by body_builder.dart
   // and ast_builder.dart.
-  void finishFunction(formals, AsyncMarker asyncModifier, body) {
+  void finishFunction(covariant formals, AsyncMarker asyncModifier, covariant body) {
     return internalError("Unsupported operation");
   }
 
@@ -72,23 +72,23 @@ abstract class StackListener extends Listener {
     stack.addLast(node);
   }
 
-  Object peek() {
+  T peek<T>() {

[ahe, 2017/02/02 07:28:30]

This is precisely the kind of changes that I'm very concerned about.

Generic methods will add an overhead at runtime when strong mode is fully
implemented. These methods are extremely hot and I'm doubtful that generic
methods will have the right performance characteristics.

     Object node = stack.last;
     return node is NullValue ? null : node;
   }
 
-  Object pop() {
+  T pop<T>() {
     Object node = stack.removeLast();
     return node is NullValue ? null : node;
   }
 
-  Object popIfNotNull(Object value) {
+  T popIfNotNull<T>(Object value) {
     return value == null ? null : pop();
   }
 
-  List popList(int n) {
+  List<T> popList<T>(int n) {

[Siggi Cherem (dart-lang), 2017/02/02 04:26:58]

these type-args get rid of many warnings where these APIs are used. We still
have a warning inside peek and pop, we could get rid of it by adding a cast `as
T`

     if (n == 0) return null;
-    List list = new List.filled(n, null, growable: true);
+    List<T> list = new List<T>.filled(n, null, growable: true);
     for (int i = n - 1; i >= 0; i--) {
       list[i] = pop();
     }