Make Map.keys/values Iterables.

utils/pub/version_solver.dart

Index: utils/pub/version_solver.dart
diff --git a/utils/pub/version_solver.dart b/utils/pub/version_solver.dart
index 792c5e8851ca9fc3404a65b01f073895f5a00f40..f277169ffcf9fa61b9377ad467a7476d6193c9bd 100644
--- a/utils/pub/version_solver.dart
+++ b/utils/pub/version_solver.dart
@@ -502,7 +502,7 @@ class Dependency {
   bool get isDependedOn => !_refs.isEmpty;
 
   /// The names of all the packages that depend on this dependency.
-  Collection<String> get dependers => _refs.keys;
+  Iterable<String> get dependers => _refs.keys;
 
   /// Gets the overall constraint that all packages are placing on this one.
   /// If no packages have a constraint on this one (which can happen when this
@@ -537,7 +537,7 @@ class Dependency {
     for (var ref in refs) {
       if (ref is RootSource) return ref;
     }
-    return refs[0];
+    return refs.first;
   }
 
   Dependency(this.name)
@@ -578,7 +578,7 @@ class Dependency {
   String _requiredDepender() {
     if (_refs.isEmpty) return null;
 
-    var dependers = _refs.keys;
+    var dependers = _refs.keys.toList();
     if (dependers.length == 1) {
       var depender = dependers[0];
       if (_refs[depender].source is RootSource) return null;