[flutter_tools] refactor license collector (#128748)

Improve debugging for flutter/flutter#128680

In the linked issue, I cannot explain how we are getting a type error. However, this refactor eliminates the null check operator (by iterating over `MapEntries` rather than the keys) in hopes that there will be a more descriptive error in the future. The correctness of this change is verified by the existing tests in https://github.com/flutter/flutter/blob/master/packages/flutter_tools/test/general.shard/license_collector_test.dart

Author: christopherfujino

packages/flutter_tools/lib/src/license_collector.dart

@@ -82,11 +82,10 @@ class LicenseCollector {
       }
     }
 
-    final List<String> combinedLicensesList = packageLicenses.keys
-      .map<String>((String license) {
-        final List<String> packageNames = packageLicenses[license]!.toList()
-          ..sort();
-        return '${packageNames.join('\n')}\n\n$license';
+    final List<String> combinedLicensesList = packageLicenses.entries
+      .map<String>((MapEntry<String, Set<String>> entry) {
+        final List<String> packageNames = entry.value.toList()..sort();
+        return '${packageNames.join('\n')}\n\n${entry.key}';
       }).toList();
     combinedLicensesList.sort();