Fix three strong mode analysis issues.

lib/src/version_union.dart

Index: lib/src/version_union.dart
diff --git a/lib/src/version_union.dart b/lib/src/version_union.dart
index ee9f657acf6d3a7b71e1f8cfa72f3b7cf3baa025..a6715145a84223dbd839ad04ee6c4f73f430b0d5 100644
--- a/lib/src/version_union.dart
+++ b/lib/src/version_union.dart
@@ -59,7 +59,7 @@ class VersionUnion implements VersionConstraint {
 
     (flattened as List).sort(compareMax);
 
-    var merged = [];
+    var merged = <VersionRange>[];
     for (var constraint in flattened) {
       // Merge this constraint with the previous one, but only if they touch.
       if (merged.isEmpty ||
@@ -132,7 +132,7 @@ class VersionUnion implements VersionConstraint {
 
     // Because both lists of constraints are ordered by minimum version, we can
     // safely move through them linearly here.
-    var newConstraints = [];
+    var newConstraints = <VersionRange>[];
     ourConstraints.moveNext();
     theirConstraints.moveNext();
     while (ourConstraints.current != null && theirConstraints.current != null) {