Small Flutter strong mode cleanup fixes. (#7825)

* Small Flutter strong mode cleanup fixes.

These are cases where strong mode down cast composite errors
generally indicated cases that would performance or correctness
issues if Flutter code was run in a strong mode VM.

* Fix Command API so that it is always in terms of Map<String,String>.

* Fix typedef

packages/flutter_tools/lib/src/vmservice.dart

diff --git a/packages/flutter_tools/lib/src/vmservice.dart b/packages/flutter_tools/lib/src/vmservice.dart
index a7dd705..50c9a55 100644
--- a/packages/flutter_tools/lib/src/vmservice.dart
+++ b/packages/flutter_tools/lib/src/vmservice.dart
@@ -147,7 +147,7 @@
   if (collection is ServiceMap) {
     return;
   }
-  if (collection is Map) {
+  if (collection is Map<String, dynamic>) {
     _upgradeMap(collection, owner);
   } else if (collection is List) {
     _upgradeList(collection, owner);
@@ -156,11 +156,11 @@
 
 void _upgradeMap(Map<String, dynamic> map, ServiceObjectOwner owner) {
   map.forEach((String k, dynamic v) {
-    if ((v is Map) && _isServiceMap(v)) {
+    if ((v is Map<String, dynamic>) && _isServiceMap(v)) {
       map[k] = owner.getFromMap(v);
     } else if (v is List) {
       _upgradeList(v, owner);
-    } else if (v is Map) {
+    } else if (v is Map<String, dynamic>) {
       _upgradeMap(v, owner);
     }
   });
@@ -169,11 +169,11 @@
 void _upgradeList(List<dynamic> list, ServiceObjectOwner owner) {
   for (int i = 0; i < list.length; i++) {
     dynamic v = list[i];
-    if ((v is Map) && _isServiceMap(v)) {
+    if ((v is Map<String, dynamic>) && _isServiceMap(v)) {
       list[i] = owner.getFromMap(v);
     } else if (v is List) {
       _upgradeList(v, owner);
-    } else if (v is Map) {
+    } else if (v is Map<String, dynamic>) {
       _upgradeMap(v, owner);
     }
   }
@@ -920,9 +920,9 @@
   @override
   bool containsKey(Object k) => _map.containsKey(k);
   @override
-  void forEach(Function f) => _map.forEach(f);
+  void forEach(void f(String key, dynamic value)) => _map.forEach(f);
   @override
-  dynamic putIfAbsent(String key, Function ifAbsent) => _map.putIfAbsent(key, ifAbsent);
+  dynamic putIfAbsent(String key, dynamic ifAbsent()) => _map.putIfAbsent(key, ifAbsent);
   @override
   void remove(Object key) => _map.remove(key);
   @override